Config from file

[realbigsean]

Issue Addressed

Resolves: #2748

Proposed Changes

clap currently makes it very difficult to overwrite cli arguments. It might be more feasible with clap v3.0.0 which is currently in beta, because that introduces the method App::mut_arg, which we might be able to use to update default argument values at runtime. And App::get_matches_mut(), which allows you to get argument matches without consuming App.

So rather than using native clap functionality, this PR creates a wrapper struct around ArgMatches, with fallback logic to check a given config file for missing args (as @paulhauner suggested) . The glaring weakness of this though, is that we lose out on clap's validation for any arguments we get from file. So far I've found that our error handling is covering a lot of this validation well anyways, but it does fail with less information that clap would. And I haven't yet tested out all of lighthouse's flags and options.

Another potential option is to move towards a declarative style like structopt, which is being rolled into clap_derive in clap v3.0.0 (clap-rs/clap#1661). This style does make it simpler to update args at runtime.

[realbigsean]

Closing in favor of #2808