Print function name when using argparse() with shell functions

[rayx]

When using argparse() to parse arguments of a shell function, it printed "bash", instead of function name, in usage message. The change fixes it.

[rayx]

Just to explain my use case...I use command line heavily. I implement frequently used operations in shell functions and run them interactively in shell. The overhead of starting Python process when running each shell function is not an issue in my case.

[nhoffman]

Merged, thanks!