Add LateX $ and $$ math support.

Fix: https://github.com/plasticboy/vim-markdown/issues/98

Follow standard LaTeX rules, only for `$`. Reasonable choice since the main target is MathJax which uses those rules.

Not implemented for `\[` and `\]` as those are harder to implement, and less "Markdownish" and conflict with link escapes. If someone wants them, go ahead and implement =)