Add plugin manipulation commands

* Add `Derelict::Plugin`
   * Contains a name and version
   * Represents a Vagrant plugin
* Add `Derelict::Plugin::Manager`
   * Install/uninstall/update plugins for a particular instance
   * Check if a plugin is installed
   * Retrieve a list of `Derelict::Plugin` objects representing currently-installed plugins
* Add `Derelict::Instance#plugins` to get a `Derelict::Plugin::Manager` for the instance