Add notebook on feature chracterization exploration w/ linked brushing.

Here's a neat example using mpld3's linked brushing to reveal clusters in the dimensions of feature characterization.

I think this could be really useful on videos with aggregated particles or busy, messy backgrounds. (In my case, I'm about to work on videos of particles in mucus from the lungs of cystic fibrosis patients.) I'm not sure I presented this in a way that is general enough. I invite your comments on the prose.

This notebook works on trackpy 0.2. The recipe is meant to be copy/pasted by our users. We *could* incorporate it into trackpy as a convenience function, but it's really more general than that. I might see if Jake is interested in putting this directly into mpld3.