save out visualizations of image segmentation

[danbjoseph]

building on what was done in #55 add an option flag to save out the segmentation results.
i think into the same folder as the images with the same filename but maybe "_segmentation_YYYYMMDD" appended?
StreetView-NatureVisibility does the original and segmentation visualization images side-by-side in one file and shows all the categories. ours would be just the segmentation viz by itself and it would be just 2 colors - for vegetation and not-vegetation (or better yet probably, just 1 color, green for vegetation and then transparent for everything else).

[ericfrench2015]

@danbjoseph per your request I took a look at this briefly and honestly couldn't find where the segmentation image is being generated in the time I allocated. I can look more carefully in 2 weeks. In the meantime, if anyone needs inspiration (I doubt it, people working on this look way more pro than me,) they can look at the draw_polygons function here https://github.com/ericfrench2015/geo_image_experiments/blob/master/src/detection_analysis_utils.py