Set different window name foreground based on activity and bell status.

Use #[default] to reset color settings on inactive window name, so it can be controlled by `*-fg` options.

Refs #248.

![screenshot on 2013-07-09 at 09 33 24](https://f.cloud.github.com/assets/35768/765702/e8dff902-e837-11e2-8183-91ef86c6407d.png)