Add simple command-line interface

[merwok]

For #42

[skorokithakis]

This needs more, for example using argparse to properly show the help text if the command is called with arguments, to show the version, etc. It also needs to be added to the scripts part of the setup.py so it gets properly installed as a command on installation.

[ghost]

If you're willing to pull in an external dependency, I've had good luck with https://github.com/google/python-fire for command-line interfaces.

[skorokithakis]

Unfortunately I think that's too heavy for a minor feature in this case.

[merwok]

In the initial feature request I was seeing this more as a bonus, like the easter eggs in the standard library (python3 -m tarfile or python3 -m http.server). If you do want this as a standalone script with docs and all, I’ll have to spend more time on it 🙂

For command-line interface, given the Python versions supported by the project, argparse is not always available but really I would just use len(sys.argv) to see if we got 0, 1 (length) or 2 (length and number) params (inspiration from pwgen: https://linux.die.net/man/1/pwgen)

[skorokithakis]

Hmm, this is true, I guess argparse is 3.2+. I guess I'm just not a fan of having utilities that don't support at least -h, but if this is an easter egg that's undocumented and not to be relied upon, I guess this is fine.