Support incremental builds & live-reloading content for Contentful via Contentful Sync API

Currently the Contentful source plugin doesn't support incremental builds but refetches all the data on every build and implements live-reload via polling all content.

We should add support for incremental builds & live-reloading content for Contentful via the [Contentful Sync API](https://www.contentful.com/developers/docs/concepts/sync/)