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
Can you add some more details in this description too - perhaps the same text as in lines 14-18 above - I think the note about what metrics are collected should be mentioned here. Also, can you add a note about what the instance count means (i.e. it is not split by instance type)?