added Konsole-like 24-bit color control sequences

Implements escape sequences for 3-byte color mode: https://code.google.com/p/iterm2/issues/detail?id=218

From my tests, it only uses an additional 3 MB of memory or so when the output almost entirely consists of random colors as below. If the feature is unused, it only slows performance a tiny bit due to the increased size of screen_char_t.

Here's a quick screenshot of how it looks with random colors: ![image](https://f.cloud.github.com/assets/1987615/760133/380cada0-e7a6-11e2-9655-af494a6e2f2e.png)

More details on usage: https://github.com/robertknight/konsole/blob/master/user-doc/README.moreColors