Enable user to specify custom colormaps?

[pllim]

Inspired by PyAstro16 talk about Making Data Pretty by Michele Bannister (https://www.youtube.com/watch?v=dKbf3gaX-Pg) and this tweet, perhaps it is nice to let people use whatever colormaps they want. I, for one, am also curious what various astronomical objects would look like in a Wes Anderson color palette. Something to think about...

[ejeschke]

If it can be made into a matplotlib compatible color map, we can import it. Might be nice to add some documentation in the manual about how to do that. Currently I think the best way would be via ginga_config.py, but there might be a way to do something in the future with a config file.