Remote library client

This is a subtask of the next-generation web interface effort (#718).

As the new HTTP API (#736) evolves, it would be useful to start designing a command-line client for the API. Ideally, it would be possible to run some familiar beets commands (`beet ls`, etc.) against a remote library that's running a server.

In addition to being cool and useful, this will help us make the API "complete" in the sense that local operations have a remote equivalent.

Read the linked next-generation web interface issue (#718) and HTTP API issue (#736) first, then inspect how existing commands such as `beet ls` are organized. The scope is complete when a command-line client can perform familiar library operations against a server, but the issue does not define the initial command set or implementation entry point.

Written by the indexing model from the issue text.