Per-module extension flags and language listing.

Any extensions that are not enabled by a used language (Haskell2010
&c) will be shown. Furthermore, any implicitly enabled are also going
to be shown. While we could eliminate this either by using the GHC API
or a dirty hack, I opted not to: if a user doesn't want the implied
flags to show, they are recommended to use enable extensions more
carefully or individually. Perhaps this will encourage users to not
enable the most powerful flags needlessly. Enabled with show-extensions.