Support for completion generation

[vorner]

Deep below spirit, there's clap and it allows for generating completions for various shells: https://docs.rs/clap/2.32.0/clap/struct.App.html#method.gen_completions.

It would be nice if this functionality got routed through all the levels and made available to users of spirit too. This likely only means somehow exposing the internal Opts wrapper ‒ we want the suggestions to contain all the valid parameters.