This replaces (now deprecated but still functional) options
`--mathml`, `--mathjax`, `--gladtex`, `--katex`, `--webtex`.
The `plain` style can now be specified explicitly.
In defaults files, `html-math-method` is now
`math-method` (though `html-math-method` will still work).
Text.Pandoc.App.Opt:
- In unexported type CompletionKind, add MathMethods.
- In Opt, change `optHTMLMathMethod` to `optMathMethod`. [API change]
Text.Pandoc.Options:
- Rename HTMLMathMethod type to MathMethod. [API change]
- Rename `writerHTMLMathMethod` field of WriterOptions to
`writerMathMethod`. [API change]
Text.Pandoc.App.Completion:
- Clean up code.
- Add support for `--math-method`.
We previously did this only with raw blocks, on the assumption
that math environments would always be raw blocks. This has changed
since we now parse them as inline environments.
Closes#3816.
John MacFarlane