Don't outline boundaries by default

mark_boundaries used to draw outlines by default. This was necessary
partly because of the asymmetric way boundaries were being drawn. Now,
boundaries are either thick or drawn around background or drawn with
subpixel precision, so I suggest that outlines are less needed.

skimage/segmentation/boundaries.py

@@ -169,7 +169,7 @@ def find_boundaries(label_img, connectivity=1, mode='thick', background=0):
 
 
 def mark_boundaries(image, label_img, color=(1, 1, 0),
-                    outline_color=(0, 0, 0), mode='outer', background_label=0):
+                    outline_color=None, mode='outer', background_label=0):
     """Return image with boundaries between labeled regions highlighted.
 
     Parameters