Add Syntax Highlighting

[mikeselander]

We tend to write up a lot of code in dev., and in quite a few other H2s. Adding syntax highlighting to H2 would enable these code blocks to be much more readable and usable.

[rmccue]

For reference, Red Book (the engineering handbook theme) uses Prism: https://github.com/humanmade/red-book/blob/master/src/Content.js

[mattheu]

Also used prism on human made website

[CoreyBenefiel]

5