annotate with two different colors based on threshold

I added the capability to annotate an image with features of two different colors. I found this useful for playing around with cutoff thresholds. e.g. see how many features you miss if you increase the minimum size.
I know the train's already left the station, but I don't see how this test wouldn't always evaluate to True. Did you mean to write `if not isinstance(split_thresh, Iterable)`?

In any case, a string also counts as iterable, so there has to be another way.

Sorry I've been doing nothing but writing comments. I'll be back on my development computer by tomorrow, I promise :).