Add extra-ghci-libraries support to .cabal files

As stated [here][http://www.haskell.org/ghc/docs/7.8.2/html/users_guide/ghci-invocation.html] in the "Extra libraries" section, GHCi on windows resolves extra libraries by just appending .dll extension to their name and ignoring an import library contents that were linked during a build. This raises issues when dll name doesn't match an import library one. To overcome that ghc introduced an extra-ghci-libraries parameter a long time ago.

It turned out that Cabal aready knows about an existance of that parameter in its InstalledPackageInfo data type. But it always uses empty list for that parameter value so it was somewhat useless.

The proposed patch adds a new field "extra-ghci-libraries" to .cabal files and passes that value right to the InstalledPackageInfo during the build.

Some relevant links:

This guy had the same troubles as I did: http://www.haskell.org/pipermail/libraries/2007-November/008641.html
Here is suggested workaround for cabal not supporting extra-ghci-libraries: https://groups.google.com/forum/#!topic/fa.haskell/eH_FbkEz06M