main.go should have better output

[dbmurphy]

So I am working on a better password handler for the go replay system and notice the --help output is painful.

I propose we switch to something a little nicer.
OLD:

NEW:

NEW with "real" sub-command:

This was just a mockup with kingpin, but use groups and subcommands seems like it would be nicer , since so many options are for real vs stress testing.

These were added by my patch, so dont worry about them yet ;)
--auth Enable Auth mode
--authdb="admin" Database to authenticate against! If --auth used.
--username=USERNAME Username to authenticate with if --auth used.
--password=PASSWORD Password to authenticate with if --auth used.