Adds support for kchmck/vim-coffee-script plugin

This simply will load the ftplugin from [vim-coffee-script][1] via the runtime
command.

The unfortunate situation is that the master branch does not support the
--literate flag yet. And changing the global variable
`g:coffee_compiler` is a bad idea.

[Pull Request 111][2] has been submitted to address this issue. Once pushed
upstream these two plugins will work well together.

[1]: https://github.com/kchmck/vim-coffee-script
[2]: https://github.com/kchmck/vim-coffee-script/issues/111