Allow custom formatting post values based on type

Summary:
Allow custom formatting on post values based on type.
For example allow post point data to be formatted as `{lat: 1, lon: 2}`
rather than in WKT.

This was possible before but broken we refactoring in D216

Test Plan: `./bin/behat -c application/tests/behat.yml application/tests/features/posts/api.geojson.feature` tests most of this - but still breaks because the `bbox` is missing from the response after refactor.

Reviewers: shadowhand

Reviewed By: shadowhand

Projects: #ushahidi_platform

Maniphest Tasks: T438

Differential Revision: https://phabricator.ushahidi.com/D260