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) =>`
move coffee spec specific Switch definitions into separate block
Merge pull request #5 from watsoncj/coffeescript-switch-definitions

switch.vim definitions for CoffeeScript