Add support for mhchem (KaTeX extension)

[fgtham]

KaTeX provides the mhchem extension for typesetting chemical formulas. This PR adds the JS asset for KaTeX v011.1 and loads the file right after katex.min.js. I hope that's all there is to it :)

[gsantner]

Thx. I happily merge. However I don't use KaTex on my own.

There are some issues open about KaTex at Markor and nobody has stepped up to do something about it so far. As you seem to have some idea about it, it would be very cool if you can also check those? #1389 #1393

I see there are other KaTex extensions as well. Do you see any other of them as something you would use regular?

[fgtham]

There are some issues open about KaTex at Markor and nobody has stepped up to do something about it so far. As you seem to have some idea about it, it would be very cool if you can also check those? #1389 #1393

Sure, I will have a look.

I see there are other KaTex extensions as well. Do you see any other of them as something you would use regular?

There seem to be two other extensions: Copy-tex and math/tex Custom Script Type. I am not familiar with either one.

[gsantner]

Thanks for your First-time contributon, merged