Support for urgency levels

```
% notify-send --help|grep urgency
  -u, --urgency=LEVEL    Specifies the urgency level (low, normal, critical).
%
```

My suggestion would be to allow the user to override gui settings in urgency sections in twmn.conf. E.g.:
```
[urgency_critical]
background_color=#f00
duration=60000
```