Swift Argument Parser for the CLI

[liamrosenfeld]

Summary

Uses the new official Swift Argument Parser for the CLI. Allows for a simpler, more maintainable CLI.

Review Checklist

• [x] I have responded to reviews and made changes where appropriate.
• [x] I have tested the code
• [x] I have updated comments / documentation related to the changes I made.
• [x] I have rebased my PR branch onto xi-mac/master.

[liamrosenfeld]

I renamed them 👍

[cmyr]

Thanks!

[liamrosenfeld]

Thanks for a great review process!

[jeremywiebe]

Yay! Thanks @liamrosenfeld