text: Title: Radviz colors - Support Colormap and colors arguments
  Body: Currently Radviz does not support adding a custom color palette. 
  
  Here's the segment of code to change / improve :
  https://github.com/DistrictDataLabs/yellowbrick/blob/c5b2346c03ba5b8aaadf6a4037b1939e9f3063a2/yellowbrick/features/radviz.py#L176-L183