Log Message:
(textproc/R-markdown) Updated 1.5 to 1.7

# CHANGES IN markdown VERSION 1.7

- The `file` argument of `mark()` will be treated as a file path only
  if the file exists and the value is not wrapped in
  `I()`. Previously, it would be treated as a file path when it has a
  file extension, which could lead to confusing errors like #100
  (thanks, @LukasWallrich).

- When there are emojis in the text, `mark()` may fail to identify and
  embed web resources (thanks, @tdhock, yihui/knitr#2254).

# CHANGES IN markdown VERSION 1.6

- Added support for footnotes, fenced `Div`s, section numbers, `{}`
  attributes for images/headings/fenced `Div`s, and appendices. See
  `vignette('intro', package = 'markdown')` for details.

- A lot of enhancements to the HTML slides format. See
  `vignette('slides', package = 'markdown')` for details.

- Added `vignette('article', package = 'markdown')` to demonstrate how
  to write an HTML article.

- If the input to `mark()` is a file, the output will also be a file
  by default. Previously the output would be text. If you want
  `mark()` to return text output when the input is a file, you may
  specify the argument `output = NULL`.

- The Markdown option `base64_images` has been renamed to
  `embed_resources`. This option can take two possible values,
  `"local"` and `"https"`, meaning whether to embed local \ 
and/or web
  (https) resources. You can specify none, either, or both of
  them. See `vignette('intro', package = 'markdown')` for details.

- Removed the option `standalone` from the list of Markdown
  options. Please use the argument `template = TRUE/FALSE` of `mark()`
  instead. The option `standalone = TRUE` was equivalent to `template
  = TRUE`.

- Added the option `auto_identifiers` (enabled by default) to
  automatically add IDs to headings, e.g., `# Hello world!` will be
  converted to `<h1 id="hello-world">Hello world!</h1>`. \ 
You can
  certainly override the automatic ID by providing an ID manually via
  the `{#id}` attribute, e.g., `# Hello world! {#hello}`.

- Renamed the `mathjax` option to `js_math` to allow for other JS math
  libraries. The default library was changed from MathJax to KaTeX. To
  continue using MathJax, you may set `js_math: mathjax`.

- Removed the option `mathjax_embed` from the list of Markdown
  options. To embed the MathJax library, enable `"https"` in the
  `embed_resources` option instead. Note that only MathJax v3 can be
  partially embedded, and lower versions cannot.

- Renamed the option `highlight_code` to `js_highlight`, and added
  support for an alternative syntax highlighting JS library Prism.js,
  which became the default. To continue using the old default
  `highlight.js`, you may set the `js_highlight` option to
  `highlight`.

- The default version of MathJax has been changed from v2 to v3.

- The default version of highlight.js has been changed from 11.6.0 to
  11.7.0, and the default style has been switched from `github` to
  `xcode`.