Allow directories in cabal data-files (#713)

This commit adds the capability to specify a whole directory which
contains data-files in a project's .cabal file.

If a data-files entry ends with a slash, like "static/", then it will
pull in all files under the directory named "static/".

Notes:

* This has only been tested by calling matchDirFileGlob in GHCi.
* I'm not sure about the name (data FileGlob = DirTrailingSlash --
  perhaps it should have Glob on the end like the others?) but I've been
  unable to come up with anything better.