Implement automatic sorting of language pragmas

[mrkkrp]

Not to forget. I'll skip this in current PR about comments, but eventually such a feature would be good.

[mrkkrp]

Also this should fix position of the block of extensions, e.g. between module docs (if present) and module header (the thing with export list). Also take care of spacing, i.e. there should be a blank line between different sections fore readability. So far I'm skipping this sort of fine tuning.

[mrkkrp]

Also, this should include, from #17:

Splitting the {-# LANGUAGE … #-} pragmas with list of extension into pragmas with a single extension.

[ethercrow]

This should also unify the case, since GHC happily accepts both language, LANGUAGE and even LaNgUaGe.