Update optparse to argparse

[andmis]

optparse is deprecated since Python 3.2: https://docs.python.org/3/library/optparse.html

This PR updates the optparse code to use argparse, as well as to use the variable naming conventions that are typically used with argparse code. See: https://docs.python.org/3/library/argparse.html#upgrading-optparse-code

Also: you had one function called parse_args which (1) built a parser, and (2) invoked the parser to actually read the command line args, and then execute some follow-up logic. I broke this into two functions.

Some context

I have a simple script for automatically-generating fish completions for commands implemented in Python. I was using it to generate nodeenv completions for fish (submitted in this PR). Since most Python commands use argparse, and optparse is deprecated, I went ahead and bumped nodeenv to argparse.

Notes

For long arguments --like-this=some_value, using the equals sign is optional: https://stackoverflow.com/questions/14567289/equals-sign-in-pythons-argument

I understand if you don't want to merge this upstream for whatever reason (inconvenience, not worth it for you, etc.), but I figured I'd submit in case you like the bump to argparse.

[ekalinin]

Thanks!