If log.SetPrefix() contains square brackets it breaks filtering.

```
log.SetPrefix("[ this will break your filtering ]")
```

My existing code added some extra data using SetPrefix which included square brackets. When I added logutils I noticed that it was not filtering out any log messages (I got them all regardless of their level). I changed from square to normal brackets in my SetPrefix call and it fixed it.

I don't know if you need to fix this in logutils as such, but it may be worth noting it in the docs to help others avoid having this enigmatic problem.