Support a public interface to make your own completion code

[kdar]

My use case here is I'm making a debugger for an emulator, and I use clap as my command handler and linefeed as my readliner. Linefeed has a way to do tab completion in it, so it would be nice if I could get at all the subcommand/options/whatever from clap.

[kbknapp]

I'm not against this, it just may be a while before I can get to something like this. So I'll keep it in the issue tracker for implementation, but need to knock out some other items first.

In the meantime, if you look at src/completions.rs there's not inherently private about how it's done, but exposing those methods at this time would be a messy API. Namely the get_*_for_path() methods are how it's finding options, subcommands, etc. So one could almost copy/paste that into user code and get same affect until I'm able to get a good API for this.

[kbknapp]

@kdar see also #568 as I plan on working on this in the near term

[kbknapp]

I'm closing this as it's covered by #568