Support latex style literate haskell (.lhs)

[dariota]

The indentation and highlighting etc all work fine just by adding .lhs to the ftdetect (or manually setting filetype=haskell), but the comments are highlighted as if they're code. I can see it works bird style, but that's not as nice to write 😞

I would like to be able to submit a PR for this but I'm afraid I'm a total stranger to vimscript/plugins.

(edit: Just realised the highlighting is actually just the built-in highlighting - bird style suffers the same issue if you set the filetype)