We want to be able to disable colors for complement logs (since they
are likely to be opened in a text editor). There's no pressing need for
alternative log formats, but I'm interested in whether the 'pretty'
format will be easier for debugging.
I chose to add 'log_*' options rather than making a separate 'log'
section for now. There's been some discussion about trying to separate
the tracing/logging stuff into more structured sections, but that can
happen later.
This allows the error handling to be done upfront instead of for each
use. In particular, the `toml` error now points to the span of text in
the config file where the misconfigured EnvFilter value is. This is much
better than the previous error that did not indicate what was actually
causing it to happen.
* Extract into reusable function (we'll need this later)
* Merge the values we want to override over the defaults, instead of
dropping the defaults completely
* Don't unnecessarily allocate (the `vec![]` usage is gone)
