Integrate rich to make cli output pretty

[CopOnTheRun]

The current default output from treelib isn't bad, but I think it could be more visually appealing.

Things I've thought about incorporating:

• [x] Dimmed/greyed output for subvolumes that aren't reachable
• [x] Bold output for subvolumes that are mount points
• [ ] #5
• [x] Better delineation between different filesystems
• [x] Limiting long output
• [ ] Maybe emojis or something because that's what the kids like

Right now I think the easiest way to accomplish much of this would be via the rich library. While the rich library does have support for trees, its support is more for visualizing them, not interacting with them, so treelib would still be a dependency. If I had to guess, rich would just act as a layer between the library and the cli, meaning most of it's code would live in btrview.py, or maybe I'll create a separate module for it as btrview.py is getting a bit crowded.

[CopOnTheRun]

This is pretty much done. There are a few things that could still be improved upon, but the vision that I set forth when I created the issue is complete.