Migrate from Kingpin to https://github.com/urfave/cli

[Integralist]

Kingpin is unmaintained and Fastly has forked a version of it that uses an unstable branch because of the necessity to access a v3 feature set that was never actually finished. The CLI has lots of hacks in it to work around missing features.

We should migrate to a maintained parser, and I propose https://github.com/urfave/cli

Here are some docs for getting started: https://cli.urfave.org/v2/getting-started/

If I get time I'll open a PR to start experimenting with what's needed/missing and whether we need a different CLI parser.