vim: supporting multiple sources:
- vim-nox: client-server implementation without X
- latest:  latest mercurial sources (still very stable)
- default: latest release

vim-plugins: Introduce an area to put vim plugins which are worth adding to nix
because they need more effort than just "unpacking". Document that
Merge pull request #568 from MarcWeber/submit/vim-multiple-version-and-plugins

vim: supporting multiple sources & complicated plugins