Generalize pointer selection

[Fil]

A “best candidate” function receives as input the channels and max radius, and returns a function that receives the pointer position and returns the best candidate, if any.

This allows a user to build:

• a pointer that knows about rects, or links
  • we might want to build Plot.pointerRect as a standard method
• a pointer that is optimized with a quadtree or a spatial tree
• a pointer that returns several data points at the same time (maybe?)
• a pointer that knows about geojson (Plot.geo with tip: true should use Plot.centroid #1743)
• more custom behaviors?