Tab completion improvements

[woodruffw]

kbs2 currently generates tab completions via --completions=SHELL, enabling users to do something like this in their shell profile/initialization file:

eval "$(kbs2 --completions=bash)"

This is pleasant, but limited: the clap-generated completions don't support intelligent positional expansion, e.g.:

• All available record labels for <label>
• All available generators for <generator>
• Possible external subcommands (found by searching PATH for kbs2-*)

We should investigate to see whether clap can populate these correctly and, if not, how difficult it would be to implement (either in clap or on our own).

[woodruffw]

clap-rs/clap#1232 will probably enable this at some point.

[woodruffw]

Looks like clap is learning argument "hints", which will probably help with this.