MathJax support

From #100.

This implements `HOEDOWN_EXT_MATH` and `HOEDOWN_EXT_MATH_DOLLAR`. The former triggers `char_math` (from `char_escape` with `\\[` and `\\(`, or from active char `$` with `$$`), which parses the block and feed the content and opening/ending tags to the renderer callback. The latter flag enabled an extra math block syntax delimited with a single `$`. Renderer callback in `hoedown_html_renderer` outputs tags and content of the block verbatim. Not sure whether I should trim and/or collapse spaces and newlines inside the block. It’s irrelevant to MathJax.
Are you sure about using `find_emph_char` here? `find_emph_char` is designed for spans that contain Markdown, which is not the case. Using it here would mean that:

    $$ `co$$de` *foo* $$

would all be interpeted as math, when that's not true.

IMO this should have its own logic, which respects backslashes.