Accept hex-string colors in descriptor dictionaries

[TimSylvester]

Currently, addWideVectors(..., [kMaplyColor:"ff00ff"]) doesn't work, but seems like it should be an acceptable shortcut for [kMaplyColor:UIColor(hex:"ff00ff")].

[mousebird]

Ooo yeah, that would be good. I always end up building a little UIColor category when I need it.