Add new config option for logging level

Add a new configuration option at the root level:

```
log_level = "info"
```

To set the logging level. Right now, it's hardcoded:

```rust
fn config_level_or_default(log_level: &Option<String>) -> LevelFilter {
    match log_level {
        None => log::LevelFilter::Info,
        Some(level) => LevelFilter::from_str(level).unwrap(),
    }
}
```