Perfer flags to positional args

[0xazure]

The current calling convention for supernova is:

supernova <username> [<auth-token>]

I was reading a really interesting article about 12 Factor CLI Apps that builds on Heroku's 12 Factor App methodology which suggests to "prefer flags to args":

Flags require a bit more typing, but make the CLI much clearer.
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A good rule of thumb is 1 type of argument is fine, 2 types are very suspect, and 3 are never good.

We could introduce a dependency on clap to handle argument parsing, as well as to generate help docs and display version numbers.