Add term_force_256colors setting.

This patch adds a term_force_256color option. I'd agree with requiring a proper TERM env variable, however this is the only way to get Konsole to display bold text without making it bright.
![konsole-irssi](https://cloud.githubusercontent.com/assets/8402422/3871044/927f188e-20eb-11e4-85dc-607e933c5ca3.png)