Bichromatic colormap for real2rgb

Takes 2 RGB colors and returns a map or truecolor image. Same options as the colormaps from real2rgb

Based on real2rgb and the associated private functions. See Oliver Woodford's submission.

Instead of making a map for every unique combination of 2 colors I made 1 function that takes 2 colors as input.

E.g. map = bichromatic(cols);
See comment section in code for specifics