Add support for Visitor Geolocation

[lindseydiloreto]

Once we have proximity search support in place, it will become more important for the plugin to also handle visitor geolocation.

Ideally, we will rely on any geolocation tools that Mapbox may already provide. But if absolutely necessary, we will integrate with ipstack to perform the geolocation.

We have no specific ETA for this, but will report back here with any updates.