Update label2rgb doc
Split label2rgb into two related functions
Add background label handling for label2rgb_avg
Add docstring for _label2rgb_avg
Add default values for background in _label2rgb_avg
Add docstring for _label2rgb_overlay
Add tests for label2rgb avg mode
Merge pull request #1033 from jni/label_image

Add "average segment color" method for `label2rgb`