Spring plugin

Ruby on Rails now ships with [spring](https://github.com/rails/spring) which makes common commands run much faster by preloading the environment.

This plugin automatically prepends `rails`, `rake` and `rspec` commands with `spring` when possible, just like the [bundler plugin](https://github.com/robbyrussell/oh-my-zsh/blob/master/plugins/bundler/bundler.plugin.zsh) does.