Add stages to the ursula command

The objective here is to allow a user to implement both a predeploy
stage and a postdeploy stage. All the user needs to do is create
either a {ENV}/playbooks/[pre|post]deploy.yml playbook.

This feature would allow users to augment ursula to thier liking.
Merge pull request #172 from craigtracey/ursulastage

Add stages to the ursula command