Add support for auto-completion

[fwcd]

It would be cool if we could auto-complete paths. This would, however, require support for asynchronous completion handlers, which rustyline is currently missing. The rustyline-completion contains some experiments in this direction, but doesn't work yet because blocking on Tokio tasks from synchronous contexts seems to be non-trivial.

See also:

• Add an async interface kkawakam/rustyline#126
• is there a way to do autocomplete with suggestions? zyansheep/rustyline-async#15