Rework the configuration infrastructure

[ctron]

This PR reworks the configuration infrastructure inside of trunk.

The main topics are:

• Split up stored configuration model (toml) and CLI model (clap)
• Have the CLI override part of the configuration model
• Allow re-using clap's env feature instead of envy, as the env-var overrides aligns more with CLI arguments rather then with the stored configuration
• Support alternate configuration formats
• Warn about future deprecation in the configuration: there are a few cases where either the configuration outgrew itself, or there had been inconsistencies because of the unified model for config-file/cli-args. Trunk warns about that and suggests migrations.

What's missing:

• loading from Cargo.toml: that is prepared, but not implemented yet
• have all tests work: most of them work, some of them don't compile yet
• Ensure that all example configurations still work
• find a better way to internally migrate deprecated fields: right now that happens inside each sub-command, I think that's the wrong place.

[simbleau]

Ping me when it's in a test-able state and I'd love to give this a try!

[ctron]

@simbleau I guess now there's something for you to take a look at :) … Loading from Cargo.toml is implemented. There are no additional features yet (like profiles and stuff). But, it's a start. I also added an example cargo-manifest to show how it works.