Support `log` configuration in Taskwarrior

[djmitche]

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.