Replace custom CLI with argparse

[hugovk]

There's a lot of custom code for the CLI handling, which we can replace with argparse.

This saves us about 150 lines of code, and we get colour help, plus I've added backticks in some help messages for "code" formatting as well.

Right now, blurb has both subcommand and option versions of help and version:

blurb help
blurb -h
blurb --help

blurb version
blurb -V
blurb --version

This PR deprecates the subcommands in favour of the more ususal options.