patch for rxvt-unicode-256color termcap

As rxvt-unicode now supports 256 color it also adds a new termcap name "rxvt-unicode-256color" for this. I did a little patch that makes this and also termcap "xterm-256color" recognized as xterms by config option XTermTheme.
I don't know if this is the right place to suggest this? However, here is the small patch:

Committed as r2315: Added "XTerms" configuration option.