preliminary version of clusters

Adresses issue #1573 . Preparation for clustering, with an example that clusters random points and prints the clusters to console. Things to be done:
1) Tests
2) Refactoring
3) Not clustering features outside the given extent
4) Recalculating clusters on zoom/extent change
5) Clicking on cluster should zoom to the extent of the clustered features
6) (maybe add support for more clustering algorithms?)

Any other suggestions are welcome.
> I'm not an export on the build system

indeed! an amusing little typo :-)

It looks like nothing's being exported at all at the moment. Exportable methods need @api - see apidocs/readme. Options need to be listed in olx; rebase on current master, and then adapt an existing entry