litedown v0.3

• Added a new engine md to output Markdown text both verbatim and as-is, which can be useful for showing Markdown examples, e.g.,

  ```{md}
  You can see both the _source_ and _output_ of
  this `md` chunk.
  ```
  
  You can also use `{md} the engine **inline**`.

• Added a new engine mermaid to generate Mermaid diagrams, e.g.,

  ```{mermaid, fig.cap='A nice flowchart.'}
  graph TD;
      A-->B;
      A-->C;
      B-->D;
      C-->D;
  ```

• Added helper functions pkg_desc(), pkg_news(), pkg_citation(), pkg_code(), and pkg_manual() to get various package information for building the full package documentation as a single-file book (thanks, @jangorecki @llrs #24, @TimTaylor #22).

• LaTeX math environments such as equations can be numbered and cross-referenced now (thanks, @hturner, #32).

• Section headings containing the class name "unlisted" will be excluded in the table of contents.

• Provided a way to write <span> with attributes based on empty links, i.e., [text](){.class #id ...}. The empty URL here tells mark() to treat the link as a <span> instead of <a>.

• Added back/forward/refresh/print buttons to the toolbar in the litedown::roam() preview interface.

• Changed the behavior of .Rmd and .R file links in the litedown::roam() interface: previously, clicking on an .Rmd or .R filename will execute them; now it will only show their content, because fully executing the code may be expensive or even dangerous (especially when the files were not authored by you). A new "Run" button has been provided in the interface, on which you can click on to run a file in memory and preview it (i.e., the old behavior of clicking on filenames). You should use this button only if you trust the file.

• Added the JS asset @mathjax-config to enable equation numbering by default when the JS math library is set to MathJax (thanks, @hturner, #32).

• Set options(bitmapType = 'cairo') in fuse() if capabilities('cairo') is TRUE, which will generate smaller bitmap plot files (e.g., .png) than using quartz or Xlib, and is also a safer option for fuse() to be executed in parallel (rstudio/rmarkdown#2561).

• Added a new vignette engine litedown::book to make it possible to build multiple vignettes into a book. To use this engine, declare \VignetteEngine{litedown::book} only in the book index file (e.g., index.Rmd) but not in other book chapter files.

• Added support for an array of multiple authors in the YAML metadata (thanks, @AlbertLei, #28). If the author field in YAML is an array of length > 1, each author will be written to a separate <h2> in HTML output, or concatenated by \and in LaTeX output. Note that you can also write multiple authors in a single string (e.g., author: "Jane X and John Y") instead of using an array (author: ["Jane X", "John Y"]), in which case the string will be treated as a single author (they will be put inside a single <h2> in HTML output).

• Fixed the bug that the leading -, +, or * in a LaTeX math expression was recognized as the bullet list marker, which would invalidate the math expression (thanks, @hturner, #33).

• Changed the first - to : in automatically generated element IDs, including section, figure, and table IDs, e.g., the ID sec-intro-methods is changed to sec:intro-methods, and fig-nice-plot is changed to fig:nice-plot. You can still use - when manually assigning IDs to elements, e.g., # Intro Methods {#sec-intro-methods}. For backward compatibility, cross-references using - will be resolved if the : version of the ID can be found, e.g., @sec-intro-methods will be resolved to @sec:intro-methods if the former cannot be found but the latter can.

• Fixed a bug that when LaTeX math environments are written in raw LaTeX blocks (i.e., ```{=latex}), mark() will not load the math JS library such as MathJax or KaTeX unless $ $ or $$ $$ expressions are present in the document.

• As-is output accepts attributes via the chunk option attr.asis now. If provided, as-is output will be wrapped in a fenced Div with these attributes.

• Numeric output from inline code will no longer be formatted if the value is wrapped in I().

• The prefix for the automatic IDs of h1 headings has been changed from sec: to chp:. For other levels of headings, the prefix is still sec:.

• Provided a new option embed_cleanup to clean up plot files that have been embedded in HTML output (thanks, @TimTaylor, #16).

• fuse() supports the output format litedown::markdown_format now, which generates the intermediate Markdown from R Markdown without further rendering Markdown to other formats. Using this output format is equivalent to fuse(..., output = '.md') or fuse(..., output = 'markdown') (thanks, @mikmart, #35).