Add support for spring

From the Spring README:

>Spring is a Rails application preloader. It's trying to solve the same problem as spork, zeus and commands.

>I made it because we are having a discussion on the rails core team about shipping something to solve this problem with rails. So this is my proposal, as working code.

>(At least I hope it's working code, but this is alpha software at the moment. Please do try it and let me know if you hit problems.)

https://github.com/jonleighton/spring

Yes It's still alpha but I've been using for a few days without any problems and would love to see it in vim-vroom. Also, this is my first pull request ever (yay) so I would appreciate any feedback if I'm doing stuff *wrong*. :heart: