Math inside literal

[cben]

Option to not render math in some contexts, e.g. .cm-literal used by markdown.js for literals and code blocks.

Would be better handled by having a mode tell us where there are formulas (#13) but for now the workaround is easy enough to be worth doing.
And even if I fix markdown.js to understand math, it'd still be useful to have defineMathMode() and have it support some blacklisted contexts.