Currently latex rendering works for markdown cells and latex output.
However, HTML output containing latex formulas will not be processed until a true latex content is detected (or when entering a markdown cell). Given that HTML output may well contain formulas (e.g a table whose columns are latex variables), it seems natural to call the mathjax renderer after html output too. This is what this patch does.
It looks like Mathjax is caching already computed results, so the overhead may be small even if there are many formulas around.
Latexify formulas contained in html text.
This seems perfectly sensible to me, thanks! Unless @ellisonbg knows a reason that this would have unsavory side effects, we should go ahead with it.