support python -m

[kurtbrose]

e.g. python -m pip

[kurtbrose]

Related to that use case, also want to accept additional pip arguments from the user at the command line. e.g. python -m pypier install foo bar baz -> python -m pip install foo bar baz

Perhaps this doesn't fit the seashore model, but the ability to say "beyond this point are unevaluated strings, pass them through as-si" as part of the API.