Use click for the CLI

[moggers87]

While our current CLI is much better than the previous offering (you know, actually looking remotely like something users might find in other command line tools), code-wise it's a Lovecraftian horror.

After using click for another project, I think that might be the way to go. I'd like to do this without making any changes to the CLI itself too, so these changes could go into a 3.x release.