Add XTERM256 color support

Creates the possibility to use 256 colors as defined by xterm256 color
names which is used by vim. And thus creates the possibility to create
more complex color themes using the same name for the colors as vim
uses.
Hm, didn't notice it before, but some strings are the same. This means that for example `DeepSkyBlue4` will always resolve to `23` reducing abilities in creating color schemes. Would it be fine to add some suffixes to duplicated strings? E.g. `DeepSkyBlue4_2` or alike, what would be more convenient for you?