Add syntax highlighting for purescript/purs code blocks in markdown.

[utkarshkukreti]

The code is based on https://github.com/JustusAdam/language-haskell. I just replaced haskell with purescript and hs with purs.

Here's a preview of what this does:

[nwolverson]

So cool! I thought about doing this some time ago but never looked into it.

(Sad that unless you're also using Github for processing the markdown the processed output may be worse than the preview...)

[nwolverson]

I have some changes to polish off upstream after which I'll release this