Command plug-in only works for pre-commit verification

[Diogo Matsubara]

Use case:

After Tarmac merges a branch, we want it to trigger an arbitrary command (in our case `juju set charmworld revno=$(bzr revno lp:charmworld 2>/dev/null)`) to update the staging instance of charmworld.

I spoke to Dobey on IRC and his suggestion was to have the command.py refactored so it could be used as a pre commit hook and a post commit hook.

As a workaround I'm going to copy the current command.py plugin, change the class name, the hook and the plugin name to act as the post commit hook command.