Add heatmap representation

[marfago]

I would be nice to have a heatmap representation as per
https://github.com/Leaflet/Leaflet.heat

[sslupsky]

Yes, this would be sweet indeed!

[wolfspyre]

yesplease!

[daniellee]

Yes, this is fully possible. Just have to make some time.

I would absolutely accept a PR if someone else found the time (wink wink).

[samhatchett]

I made an attempt at this in PR #35 - it ends up being suitable for my own use case, which is to view ~100k points with 12-character hashes on a city-scale. Not sure if it works in the general case.