Supporting conditional tile downloads

It would be good to be able to allow/disallow tile downloads from client code.  

**Example use-case**
Let's suppose that the application has a requirement to allow any network traffic ONLY when the phone has WiFi coverage. On 3G, it can only send data if the user explicitly allows it via a setting.
Currently, MapBox downloads map tiles whenever it can and this behaviour cannot be managed by the client. 

This feature would give the client the possibility to implement a condition (in the form of a block) that allows/disallows tile downloads.