FR: Export/Import custom commands

[tecnobabble]

The ability to export/import a list of custom commands for associated vagrant boxes would be helpful in both migrating from one development machine to another as well as for deploying custom commands to a large sub-set of users. Some expansion on this:

• Command line option to import/export (so it's scriptable as part of an install)
• Ability to choose between appending existing commands or overwrite

Today, it's only seemingly doable by copying plists around.