Make Completion a stateful API

[lthoerner]

I was trying to create a path completion engine using the completion API, but found that it was very difficult because Completion::get() was intended to be a stateless function, taking &self. I changed this to &mut Self, changing the interior type appropriately. Though this crate is not at 1.0 yet, if you wish not to make any breaking changes to the API, we can figure out a sort of backward-compatible solution. Let me know what you think.

[djc]

I only recently started helping out with dialoguer maintenance, so I'm unfamiliar with the subtleties of the API. I really haven't used dialoguer much at all, so pretty much I can only provide pretty passive maintenance to review/merge/publish obvious fixes/improvements.

This does not seem like an obvious fix/improvement to me -- it imposes costs on all downstreams by internalizing an Arc<Mutex<_>> that so far hasn't been needed -- and as such it seems most users so far haven't needed it.

So I don't think I'm comfortable merging this.