Log Message:
md4c: update to 0.5.1.

## Version 0.5.1

Changes:

 * LaTeX math extension (`MD_FLAG_LATEXMATHSPANS`) now requires that opener
   mark is not immediately preceded with alpha-numeric character and similarly
   that closer mark is not immediately followed with alpha-numeric character.

   So for example `foo$ x + y = z $` is not recognized as LaTeX equation
   anymore because there is no space between `foo` and the opening `$`.

 * Table extension (`MD_FLAG_TABLES`) now recognizes only tables with no more
   than 128 columns. This limit has been imposed to prevent a pathological
   case of quadratic output size explosion which could be used as DoS attack
   vector.

 * We are now more strict with `MD_FLAG_PERMISSIVExxxAUTOLINKS` family of
   extensions with respect to non-alphanumeric characters, with the aim to
   mitigate false positive detections.

   Only relatively few selected non-alphanumeric are now allowed in permissive
   e-mail auto-links (`MD_FLAG_PERMISSIVEEMAILAUTOLINKS`):
     - `.`, `-`, `_`, `+` in user name part of e-mail address; and
     - `.`, `-`, `_` in host part of the e-mail address.

   Similarly for URL and e-mail auto-links (`MD_FLAG_PERMISSIVEURLAUTOLINKS` and
   `MD_FLAG_PERMISSIVEWWWAUTOLINKS`):
     - `.`, `-`, `_` in host part of the URL;
     - `/`, `.`, `-`, `_` in path part of the URL;
     - `&`, `.`, `-`, `+`, `_`, `=`, `(`, `)` in the query part of the URL
       (additionally, if present, `(` and `)` must form balanced pairs); and
     - `.`, `-`, `+`, `_` in the fragment part of the URL.

   Furthermore these characters (with some exceptions like where they serve as
   delimiter characters, e.g. `/` for paths) are generally accepted only when
   an alphanumeric character both precedes and follows them (i.e. these cannot
   be "stacked" together).

Fixes:

 * Fix several bugs where we haven't properly respected already resolved spans
   of higher precedence level in handling of permissive auto-links extensions
   (family of `MD_FLAG_PERMISSIVExxxAUTOLINKS` flags), LaTeX math extension
   (`MD_FLAG_LATEXMATHSPANS`) and wiki-links extension (`MD_FLAG_WIKILINKS`)
   of the form `[[label|text]]` (with pipe `|`). In some complex cases this
   could lead to invalid internal parser state and memory corruption.

   Identified with [OSS-Fuzz](https://github.com/google/oss-fuzz).

 * [#222](https://github.com/mity/md4c/issues/222):
   Fix strike-through extension (`MD_FLAG_STRIKETHROUGH`) which did not respect
   same rules for pairing opener and closer marks as other emphasis spans.

 * [#223](https://github.com/mity/md4c/issues/223):
   Fix incorrect handling of new-line character just at the beginning and/or
   end of a code span where we were not following CommonMark specification
   requirements correctly.

## Version 0.5.0

Changes:

 * Changes mandated by CommonMark specification 0.30.

   Actually there are only very minor changes to recognition of HTML blocks:

   - The tag `<textarea>` now triggers HTML block (of type 1 as per the
     specification).

   - HTML declaration (HTML block type 4) is not required to begin with an
     upper-case ASCII character after the `<!`. Any ASCII character is now
     allowed. Also it now doesn't require a whitespace before the closing `>`.

   Other than that, the newest specification mainly improves test coverage and
   clarifies its wording in some cases, without affecting the implementation.

   Refer to [CommonMark
   0.30 notes](https://github.com/commonmark/commonmark-spec/releases/tag/0.30)
   for more info.

 * Make Unicode-specific code compliant to Unicode 15.1.

 * Update list of entities known to the HTML renderer from
   https://html.spec.whatwg.org/entities.json.

New Features:

 * Add extension allowing to treat all soft break as hard ones. It has to be
   explicitly enabled with `MD_FLAG_HARD_SOFT_BREAKS`.

   Contributed by [l-m](https://github.com/l1mey112).

 * Structure `MD_SPAN_A_DETAIL` now has a new member `is_autolink`.

   Contributed by [Jens Alfke](https://github.com/snej).

 * `md2html` utility now supports command line options `--html-title` and
   `--html-css`.

   Contributed by [Andreas Baumann](https://github.com/andreasbaumann).

Fixes:

 * [#163](https://github.com/mity/md4c/issues/163):
   Make HTML renderer to emit `'\n'` after the root tag when in the XHTML mode.

 * [#165](https://github.com/mity/md4c/issues/165):
   Make HTML renderer not to percent-encode `'~'` in URLs. Although it does
   work, it's not needed, and it can actually be confusing with URLs such as
   `http://www.example.com/~johndoe/`.

 * [#167](https://github.com/mity/md4c/issues/167),
   [#168](https://github.com/mity/md4c/issues/168):
   Fix multiple instances of various buffer overflow bugs, found mostly using
   a fuzz testing. Contributed by [dtldarek](https://github.com/dtldarek) and
   [Thierry Coppey](https://github.com/TCKnet).

 * [#169](https://github.com/mity/md4c/issues/169):
   Table underline now does not require 3 characters per table column anymore.
   One dash (optionally with a leading or tailing `:` appended or prepended)
   is now sufficient. This improves compatibility with the GFM.

 * [#172](https://github.com/mity/md4c/issues/172):
   Fix quadratic time behavior caused by unnecessary lookup for link reference
   definition even if the potential label contains nested brackets.

 * [#173](https://github.com/mity/md4c/issues/173),
   [#174](https://github.com/mity/md4c/issues/174),
   [#212](https://github.com/mity/md4c/issues/212),
   [#213](https://github.com/mity/md4c/issues/213):
   Multiple bugs identified with [OSS-Fuzz](https://github.com/google/oss-fuzz)
   were fixed.

 * [#190](https://github.com/mity/md4c/issues/190),
   [#200](https://github.com/mity/md4c/issues/200),
   [#201](https://github.com/mity/md4c/issues/201):
   Multiple fixes of incorrect interactions of indented code block with a
   preceding block.

 * [#202](https://github.com/mity/md4c/issues/202):
   We were not correctly calling `enter_block()` and `leave_block()` callbacks
   if multiple HTML blocks followed one after another; instead previously
   such blocks were merged into one.

   (This may likely impact only applications interested in Markdown's AST,
   and not just converting Markdown to other formats like HTML.)

 * [#210](https://github.com/mity/md4c/issues/210):
   The `md2html` utility now handles nested images with optional titles
   correctly.

 * [#214](https://github.com/mity/md4c/issues/214):
   Tags `<h2>` ... `<h6>` incorrectly did not trigger HTML block.

 * [#215](https://github.com/mity/md4c/issues/215):
   The parser incorrectly did not accept optional tabs after setext header
   underline.

 * [#217](https://github.com/mity/md4c/issues/217):
   The parser incorrectly resolved emphasis in some situations, if the emphasis
   marks were enclosed by punctuation characters.