convert ansi color names to 256color indexes

References:
    * http://en.wikipedia.org/wiki/ANSI_escape_code#Colors
    * http://vim.wikia.com/wiki/Xterm256_color_names_for_console_Vim
    * https://github.com/bukzor/dotfiles/blob/master/bin/colortest256

Closes issue #4

These color look ok, but you may want to tweak them further.
I've taken liberty with "yellow/brightyellow" and made it "brown/tan".