Barebones CLI for pm

I wrote this code without thinking very clearly. I should have seen that there is already a Go client library for the HTTP API and that types are already defined. But I'm pushing it because it's better than nothing, and it illustrates the base behavior I want.

Things that need to be fixed before this is worth considering IMO:

- We need to use commandline flags to make some of the variables settable.
- We should use the included types and the Go client library.

Things I would like to improve after that:

- Auto-detect the screen size.
- Make the client library support a timeout if requests hang.
- Support basic keystroke commands; these are documented in issues already.