Log Message:
md4c: update to 0.5.2.

Changes:

 * Changes mandated by CommonMark specification 0.31:

   - The specification expands set of Unicode characters seen by Markdown
     parser as a punctuation. Namely all Unicode general categories P
     (punctuation) and S (symbols) are now seen as such.

   - The definition of HTML comment has been changed so that `<!-->` and
     `<!--->` are also recognized as HTML comments.

   - HTML tags recognized as HTML block starting condition of type 4 has been
     updated, namely a tag `<source>` has been removed, whereas \ 
`<search>`
     added.

   Refer to [CommonMark 0.31.2](https://spec.commonmark.org/0.31.2/) for full
   specification.

Fixes:

 - [#230](https://github.com/mity/md4c/issues/230):
   The fix [#223](https://github.com/mity/md4c/issues/223) in 0.5.1 release
   was incomplete and one corner case remained unfixed. This is now addressed.

 - [#231](https://github.com/mity/md4c/issues/231):
   `md2html --full-html` now emits `<meta  charset="UTF-8">` in \ 
the HTML header.