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/
Sync of https://github.com/pantsbuild/pants/commit/c5150bf459373c9dba9d82419464b59e1413b1c6