Generalised all functions in Parsing.hs

Before it wasn't possible to use these general combinators with the ParsecT
transformer but with the more general types this is now possible.
Removed (>>~) function

This function is equivalent to the more general (<*) which is defined in
Control.Applicative. This change makes pandoc code easier to understand for
those not familar with the codebase.
Removed inline fmap from Parsing.hs

Replaced all inline occurences of fmap with the more idiomatic (<$>).
Merge pull request #1414 from mpickering/general

Improvements to Parsing.hs