Distinguish pragmas from comment blocks

This is how I fixed #10. It also works for these cases (from the Haskell documentation):

    module Wibble {-# DEPRECATED "Use Wobble instead" #-} where
      ...

    data T = T {-# UNPACK #-} !Float
               {-# UNPACK #-} !Float

not handled is this example:

    import {-# SOURCE #-} A( TA(..) )