Patch to read package-specific cabal-install config

Hi all, for packages that aren't for public consumption, we would like to specify a private Hackage repo location. The simplest way we could think of to do this was to read a config file in the same format as the user config (~/.cabal/config) from the current directory and let any flags set in that file override the user config. The attached patch implements this, reading any file with a .cabal-install extension (including '.cabal-install'). It errors out if more than one .cabal-install file is found or if there's a parse error on the .cabal-install file. Does this sound like a good idea? If not, any ideas for alternative ways of handling non-public packages? Many thanks, Carl