Use the `gh` tool if available and user is logged in with it (or just unconditionally require it)

[fingolfin]

Supersedes #102:

Telling people to install gh from https://cli.github.com and then doing gh auth login is probably superior to everything else we can do and also benefits them longterm (might have to add special instructions for people using Ubuntu through WSL on Windows, though, as I learned the hard way today)