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.
Some people (like me, sometimes) have the habit of using tuples for sequence literals. `isinstance(split_thresh, (list, tuple))` would save those folks from seeing a confusing exception.

I know I haven't said much on this PR, but I'm looking forward to this being in trackpy.