sync to an explicit hash for pyyaml

I recommended using the tag name, but I shouldn't have. Tags can change,
but hashes are forever.
add docopt to peru.yaml and third-party
reimplement the command line parser with docopt
loop over command names in main

This reduces some duplication in the big triage statement in main, but
the important benefit is that we can tell when peru is invoked without
any command, and we can print help in that case instead of printing
"peru.yaml not found".