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`.
Make `--bash-completion` an alias of `--completion=bash`.
Reorganize option parsing code using a new data structure OptionSpec,
which can encode the type of completion needed by each option.
Add new unexported module Text.Pandoc.Completion.
Unexported module Text.Pandoc.CommandLineOptions: export OptionSpec.
Closes#8542.
John MacFarlane