Include XKCD colors

It would be great if you could integrate the colors from the XKCD color survey just how they are used in matplotlib, i.e. with xkcd: prefix, e.g. xkcd:red. I would think it is best to put them in a separate list that can be toggled on/off. Here is the full list of colors: https://xkcd.com/color/rgb.txt

For testing, I had added the colors to the colors.py. That turns out to be a bad idea because it can greatly slow down Sublime upon saving files.