Convenience scale getter

[davenquinn]

This library is awesome. I am wondering if you would be interested in a convenience function for getting the scale by name. It takes a little bit to inspect the dict to get the relevant scheme. A simple function that handles the scanning and integer conversion, with some helpful error messages, would make this super-usable. Something like

cl.get_scale('Purples', 10)
cl.get_scale('PuGr') # defaults to 5 divs

I'd be happy to code something up if you were willing to merge.