Rename project to 'fern', as simple-logging isn't a very interesting or unique name.
Add the ability to use a custom (user-defined) logger implementation
Update README a bit
Move Level to api.rs, and clean up a few other imports
Clean up the structure a bit:
- Get rid of the IntoLogger trait
- Move implementations of into_logger() for *Config into config.rs
- Rename Output and Logger into ConfigOutput and ConfigLogger in config.rs, and stop renaming them when importing to lib.rs