Plugins loaded hook

As I just wrote on the [mailing list](https://groups.google.com/d/msg/vagrant-up/-1DQRhzihZo/5ssgqmVqZFsJ) about a potential "vagrant-dotenv" plugin and based on [what we had to do on Bindler](https://github.com/fgrehm/bindler/blob/master/lib/bindler/bend_vagrant.rb#L7-L16) I propose that we add a new hook to `Environment` that gets triggered after loading plugins but prior to loading `Vagrantfile`s.

This will allow plugin developers to do all sorts of "magic" before configurations from Vagrantfiles are parsed. The current [`:environment_load`](https://github.com/mitchellh/vagrant/blob/master/lib/vagrant/environment.rb#L135) hook [ends](https://github.com/mitchellh/vagrant/blob/master/lib/vagrant/environment.rb#L271) up [parsing](https://github.com/mitchellh/vagrant/blob/master/lib/vagrant/environment.rb#L513) the Vagrantfiles and we can't use that.

I hope that's enough information for this to get merged but I'd be fine to provide use cases if needed :D