(textproc/R-markdown) Updated 1.1 to 1.5

# CHANGES IN markdown VERSION 1.5

- Values of meta variables `title`, `author`, and `date` (if provided)
  will be transformed to the target output format before they are
  passed into templates.

- Fixed the bug that the default CSS was not added to HTML output.

- Removed dependency on the **mime** package.

- Added experimental support for HTML slides:
  `markdown::mark_html(..., meta = list(css = c('default', 'slides'),
  js = 'slides'))`. If you prefer knitting `Rmd` documents in RStudio,
  you may use the output format:

  ```yaml
  output:
    markdown::html_format:
      meta:
        css: [default, slides]
        js: [slides]
  ```

  See https://yihui.org/en/2023/01/minimal-r-markdown/ for a demo.

# CHANGES IN markdown VERSION 1.4

- Empty `\title{}` in LaTeX output will be removed (along with `\maketitle`).

- highlight.js is loaded from
  https://www.jsdelivr.com/package/gh/highlightjs/cdn-release by
  default now. This means more languages are supported (not only R),
  but also means syntax-highlighting will not work offline at the
  moment (it will be improved in future).

- MathJax failed to load in the previous version. The bug has been
  fixed now.

- Removed the function `markdownExtensions()`.

# CHANGES IN markdown VERSION 1.3

- Switched the underlying Markdown rendering engine from the C library
  **sundown** (which has been deprecated for a decade) to the R
  package **commonmark** (thanks, @jeroen, yihui/knitr#1329).

- The functions `renderMarkdown()` and `markdownToHTML()` have been
  renamed to `mark()` and `mark_html()`, respectively. The old names
  are still kept in this package for backward-compatibility.

- Removed the arguments `stylesheet` and `fragment.only` in
  `mark_html()`. For `stylesheet`, please use the argument `meta =
  list(css = ...)` to provide the CSS stylesheet. For `fragment.only`,
  please use `mark_html(template = FALSE)` or `mark_html(options =
  '-standalone')` instead of `fragment.only = TRUE`. Currently these
  old arguments are still accepted internally, but may be deprecated
  and dropped in the long run.

- The `file` argument of `mark()` and `mark_html()` can also take a
  character vector of Markdown text now.

- Removed functions `rendererExists()`, `rendererOutputType()`, and
  `registeredRenderer()`. They were primarily for internal use.

- Deprecated the function `markdownExtensions()`. All extensions
  should be specified via the `options` argument of functions like
  `mark()`, e.g., `mark(options = '+table+tasklist')`. See all options
  on the help page `?markdown::markdown_options`.

- Renamed `markdownHTMLOptions()` to `markdown_options()`.

# CHANGES IN markdown VERSION 1.2

- Fixed the warnings "a function declaration without a prototype is
  deprecated in all versions of C" (#94).

(mef)