Polymorphic interpolate function

Created mix interface, cloned types (can't implement methods for
nonlocal types), implemented interpolate method for cloned types using
reflection to extract colour argument.

Changed colorMap in mandelbrot.go to map to mixColor values. Changed
call to interpolate to method call syntax and return type of colorFor to
mixColor.

Also added alternative iteration escape function. I probably want to
make a more general interface to replace mandel - something like
escapeplot.