switch.vim definitions for CoffeeScript

switch.vim definitions for CoffeeScript and specs

- switch between `it`, `xit` and `it.only`
- switch between `describe`, `xdescribe`, and `describe.only`
- switch between `->`, `=>`, `(done) ->`, and `(done) =>`

bonus fix:
`:source ~/.vim/plugins.vim` no longer throws duplicate function definition error