Support specifying a partial name to limit which partials are rendered o...

I would appreciate your feedback on this. This pull allows you to specify partial as an option when calling sync_update. This is really useful in the case where a resource is comprised of many smaller sync partials and a certain action has taken that place that warrants that only a particular partial needs to be updated.

Test coverage is a little weak but can be improved once the test suite is fleshed out a little more.