Persisted environment variables for strong-pm hosted applications

 * [x] Create persisted environment on pm start
 * [x] Inject persisted environment into `prepare` and `run` phases
 * [x] Set environment variables via `pmctl env-set K=V [K2=V2...]`
 * [x] Get environment variables via `pmctl env-get [KEYS...]`
 * [x] Unset environment variables via `pmctl env-unset KEYS...`
 * [x] CLI argument parsing/validation
 * [x] Document CLI

Additional functionality for later PR:
 * Expose environment via REST API models as part of service state

Refs: strongloop-internal/scrum-nodeops#99
I notice that this only lists env vars that have been set by strong-pm. env vars already present aren't listed. It can be good to know those. What do you think? Of course, that raises the question of how you know which are strong-pm maintained, and which inherited. Maybe too thorny for now?