Support `population` and `popularity` data

[orangejulius]

Now that we are close to using popularity data in venues from OSM (https://github.com/pelias/openstreetmap/pull/531), it's even more clear that custom data can benefit from popularity scores as well.

[missinglink]

It might be worth adding a small warning to the readme that we only suggest setting either one or the other at this stage, setting both may have unpredictable results!