HTML reader:  Parse LaTeX math if appropriate options are set.

* Moved inlineMath, displayMath from Markdown reader to Parsing.
* Export them from Parsing.  (API change.)
* Generalize their types.
Text.Pandoc:  Don't default to pandocExtensions for all writers.

In particular, we don't want to default to math parsing for the
HTML writer.
Small change to HTML reader tests.

"$" is now a special character.
Documented use of extensions with non-markdown input formats.