Add post-environment update triggers

r10k needs to be able to trigger file sync service when r10k has finished synching an environment. This should be added so that users can specify either an arbitrary command to run after r10k finishes updating an environment, or alternately have a purpose built trigger for interacting with the file sync service.

There is already a "postrun" command, but that only fires when r10k has finished it's execution and does not pass any information about the environment(s) that have been deployed. I think what this ticket is talking about is being able to fire things as soon as each individual environment is done being deployed.