There are several noteable changes in this commit that mean that logging
will behave differently to how it did previously.
- There are now no separate filter levels for terminal and file output.
This is a feature that is lacking in `tracing` but is set to be resolved
by https://github.com/tokio-rs/tracing/pull/508. At which point we MAY
include it.
- The default log level was previously `WARN` - it is now `INFO`.