Add a subcommand to mrjob/tools for counting active emr jobflows and instance counts and print out collected stats

Adding a subcommand 'collect-emr-active-stats' to mrjob for counting active emr clusters/instances and print the stats

Active jobflows are those in states of BOOTSTRAPPING, RUNNING, WAITING, and STARTING

Output stats including:
- timestamp: datetime.utcnow().isoformat() 
- num_jobflows: computed by adding all jobflows in active states
- total_instance_count: summing all instance counts of active jobflows
Why making `now` an argument?