Support bincapz configuration via environment variable or file

[egibs]

As of #436, we have a much more structured way of calling bincapz with the desired configuration, but long CLI commands make certain things a little more difficult (running the bincapz CLI from within Go code, for example).

It would be nice to support configuration via environment variables (e.g., via github.com/sethvargo/go-envconfig) as well as via a config file (.bincapz or something similar).