Add option to disable just $...$ math, not \(...\) math

[jez]

Unlike pandoc itself, vim-pandoc-syntax eagerly treats everything after
a $ as inline math. The workaround for pandoc markdown documents that
make heavy use of $-based prices is to use \$, which both pandoc and
vim-pandoc-syntax recognize as a literal dollar sign.

Personally, I would rather have $ always mean a dollar sign, and use
\(...\) for inline math, because I never use \(...\) for anything
else.

With that in mind, I've added a way to disable all special behavior for
$...$ inline math.