Allow aggregating properties in turf-clusters-dbscan

[stevage]

clustersDbscan() does not allow any way to determine which source points have been grouped in each cluster, or a way to aggregate properties in that cluster.

It would be helpful if one or both of these options were supported:

• a clusterIds option, which if true would include a clusterIds property, an array of ids of each feature in the cluster.
• an aggregateFunc option, a function which would be called with a feature being clustered, and the cluster it is being grouped into.