Allow all Vagrant's SSH Settings to be set in config

[fnordfish]

Creates a new config sub-section ssh which reflects all of the current Vagrant SSH Settings.
The current username config is still available and is preferred over ssh.username.

[sethvargo]

After discussion, we think these patterns could quickly grow out of scale. Instead of handling all the special edge cases, I've created #51 to allow users to specify a full Vagrantfile which can be fully customized.