Field selection as parameter

Allow the fields that are selected to be stored to be selected via a parameter. This will allow custom selection of which fields from the Google Maps results should be stored in the contact data.
I've added it now so that if they request `'geometry.bound.northeast.lat'` it will dig into the structure, and then store it under `'location:geometry:northeast:lat'`. I'll push that and let you have a look.

Yes I think I'll do that unpacking in another PR, I'm not sure how long it will take to get it right, best to do it separately. At the moment it will just store the stringify'd version of whatever the parameter requests.