It's a bit weird to *have* to fire up a browser to get fish_config to
choose a prompt.
So this adds a `prompt` subcommand to `fish_config`:
- `fish_config prompt list` shows all the available prompt names
- `fish_config prompt show` demos the available sample prompts
- `fish_config prompt choose` sources a prompt
- `fish_config prompt save` makes the choice permanent
A bare `fish_config` or `fish_config browse` opens the web UI.
Part of #3625.
TODO: This shows the right prompt on a new line. Showing it in-line is awkward
to do because we'd have to move it to the right.
