Add LaTeX support with MathJax or MathQuill

This PR adds (optional) support for inline LaTeX with MathJax or MathQuill. Use `$$` (block) or `$%` (inline) to mark the LaTeX regions of a post, like this:

````It should be $%\frac {v - v_0} {a} = t $% rather than $%v - v_0 / a = t$%.````

TODO: this PR doesn't contain the live preview during editing. That will be part of the "replace-wmd-with-pagedown" PR.