Add `vagrant-box-updater` plugin to make further standardization possible

Wow. Had never thought to implement a plugin for this, but have definitely been bitten by the fact that, depending on when you first run vagrant, you might have vastly different basebox versions.

https://github.com/spil-ruslan/vagrant-box-updater

This plugin simply checks when a box is updated, notifies you of the modification date, and asks whether you'd like to update.