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.
Some color names were two times in the list.

Added "_2" in this case. Other than this, the color names corrspond to
this list:
http://vim.wikia.com/wiki/Xterm256_color_names_for_console_Vim
Deduplicate more of 256 colors
Document 256 color names addition

Thanks to Michael Vetter (a.k.a. jubalh).
Merge branch 'add-xterm256-color-support'

Add more human-friendly (xterm-like) names for 256 colors.

Thanks to Michael Vetter (a.k.a. jubalh).