Support for real distance on nearbyDistance function

[yvomenezes]

The nearbyDistance function currently takes into consideration only the pixel distance between the markers. Would it be possible to add support for real distance?

[jawj]

I'm not sure this is really in scope for this library. nearbyDistance specifies the pixel radius within which other markers will be spiderfied. It's in pixels, not metres, because it's about how close together the markers are on the map at the current zoom level, not in real life.