Enable passthru args for python run.

Also enabled an explicit flag for this, which we shockingly didn't have before somehow.

Made some changes to example code to allow manual testing of this. Try:

./pants goal run.py --args="benjy"  examples/src/python/example/hello/main/ -- zundel jsirois

Changed the python run task's registration name from 'python-run' to 'py',  so that the cmd-line
scope is run.py instead of run.python_run.

Also added some missing __init__.py files.

Reviewed at https://rbcommons.com/s/twitter/r/1321/
Allow more succinct invocations with passthru args.

If passthru args "belong" to a goal scope then all tasks in that goal get them.

E.g., you can do:

pants goal run <python-target> -- passthru args

and the run.py task will get the passthru args. So will run.jvm, but that's OK
because it's a no-op for <python-target>.

This obviously is only good when in practice only one passthru-arg-supporting
task in that goal is not a no-op. However that is the common case.

Reviewed at https://rbcommons.com/s/twitter/r/1323/