Add colors on the fly

[ChrisBarker-NOAA]

It would be nice to be able to add a new color on the fly:

im.draw_line(pt1, pt2, line_color = (234, 323, 123))

then that color would get added to the palette on the fly, if it wasn't already there. (we could use a string-ified version of RGB for teh name...)

Of course, then it wouldn't have a name, but it seems like this would be nice.

Overall, a two way dict for colors, so you could look them up by name, rgb values, or index...