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.
Two things:
 
Thing 1: Sorry for not catching this earlier. Custom contact data (anything that is not a standard contact field like `msisdn`, `name`, `surname`, ...) needs to be stored under `contact.extra`:

```javascript
self.contact.extra.foo = 'bar';
self.contact.extra.bar = 'baz';
```

Thing 2: JSON stringifying the data would work for jsbox apps needing to access the data, but we also need to allow people to be able to search extra fields from vumi-go's UI, so until we support storing of json data as contact extras, we'll need to do own namespacing of the fields to end up with something like:

```
self.contact.extra['location:field_a'] === 'value_for_field_a';
self.contact.extra['location:field_b'] === 'value_for_field_b';
...
```

Its a little meh needing to our own namespacing, but hopefully we will be able to store json extras at some point soon.