Layer structopt on top of config-rs

[ThomasMeier]

Structopt is used for CLI library. It takes a data structure and builds the cli handler from it.

We also use config-rs which layers configuration up from multiple sources to set up a configuration.

Ideally, then, the structopt system will put the collected information from the user to be layered on top of config-rs.

This is an evolving issue in the rust ecosystem (rust-cli/config-rs#111 sort of relevant). I expect structopt and config-rs and clap to merge or have compatible APIs. In the meantime, we simply must put the structopt results on top of the config-rs result.