Support `log` configuration in Taskwarrior

It would be great if the various output from `log::foo!()` in Rust was visible when running `task`, based on some Taskwarrior config.

I think `log` has some global configuration that can be configured once at startup. `env_logger` does so from an env var, but presumably we could also do so via an FFI call.

Ideally this would just be including `taskchampion` in the list of `verbose` values.