Add mhchem KaTeX extension to math.html

[jskherman]

Hello!

I would like to suggest adding the simple addition of the mhchem extension in the $\text{KaTeX}$ implementtion. Personally, I find the extension useful for writing units quickly (through the \pu{example unit} command) and chemical formulae (like $\ce{2H2 + O2 -> 2H2O}$) in my notes and so I thought it could be useful for others to have the functionality as well.

[ppeetteerrs]

Sure thanks! I would merge it in the next release.