Add --changelist and --changelist-format

Allow output of changed tiles to a file specified with --changelist. The
file contains the name of each tile on a line. The path can be relative
or absolute, chosen using --changelist-format.

The code assumes that all innertiles will have been updated when a
worldtile changes, and prints the corresponding filenames to the list
directly, rather than add message passing to the innertile methods.

overviewer_core/quadtree.py

@@ -479,6 +479,8 @@ class QuadtreeGen(object):
             tileimg.save(imgpath, quality=self.imgquality, subsampling=0)
         else: # png
             tileimg.save(imgpath)
+        #Add tile to list of rendered tiles
+        poi_queue.put(['rendered',imgpath])
 
         if self.optimizeimg:
             optimize_image(imgpath, self.imgformat, self.optimizeimg)