Call-for-help: Add XDGBDS support to `directory` package

[Herbert Valerio Riedel]

Hello *,

I'd like to invite everyone with knowledge of the 
XDG Base Directory Specification[1] to contribute to

  https://github.com/haskell/directory/issues/6

>From what I gathered, it appears to me that adhering to the XDGBD Spec
(on Linux at least) is desirable and "strongly encouraged" by Linux
distributions[2], so I think this ought to be implemented sooner rather
than later (maybe even in time to be part of GHC 7.10.1)

This issue was originally filed against GHC, since it affects the
location of the `~/.ghc` folder (and fwiw this would also affect cabal's
`~/.cabal` folder location).

Thanks,
  hvr

 [1]: http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
 [2]: https://wiki.debian.org/XDGBaseDirectorySpecification