math highlighting

[aaren]

Math highlighting

Math in multimarkdown is written in TeX and delimited using
\\( ..math.. \\) and \\[ ..math.. \\] for inline and display
math respectively.

Previously, the highlighting in vim-multimarkdown got confused with
math environments, especially when they included * or _.

This patch fixes this and highlights math environments as TeX.

I've also allowed demarcation of math using $$ ..math.. $$ and
$ ..math.. $ for display and inline. These can be commented out
to switch off.

Stackoverflow question