command line utility (nsq_stat)

add a command line utility that would provide similar information to `nsqadmin` like depth and other metrics for the cluster (for a given topic/channel).

the benefit would be it would provide *aggregate* stats on the command line (for those who prefer to use that kind of UI).

if the format were periodic like `iostat` it would also provide the context over time that is invaluable in identifying production issues.

cc @manover