Cabal-add: extend Cabal build-depends from the command line

…and another release, cabal-add-0.2.1. This drops a dependency on cabal-install-parsers and now uses cabal-install proper. It would be nice if someone gives the new release a spin with --project-file, as this is the functionality affected.

We used cabal-install-parsers to parse cabal.project and determine which Cabal files are listed there. Unfortunately, cabal-install-parsers fell behind Cabal-syntax and, more importantly, the syntax of cabal.project files has been recently extended with imports and conditionals, making it difficult for third-party tools to cope. I managed to use cabal-install as a library for this purpose, although its interface is far from being convenient for this task or stable across releases.