ENH: Allow one to replace the annotation functions.

[rkern]

Step 1 in my master plan for data-rich export. I was thinking about making a rich AnnotatedGraph where each edge and vertex would be annotated with a dict with more metadata (sys.getsizeof(), mro of its type, length if it has one, shape and dtype for ndarrays, etc.). Then I can output the edges and rows of the AnnotatedGraph as JSON Lines, one edge or vertex per line. Reconstructing the enriched AnnotatedGraph is straightforward; then we can do lots of queries off-line on that AnnotatedGraph rather than the ObjectGraph (e.g. find all of the Enable Components and their referrers).