Add VecTiles TopoJSON MultiProvider

It looks possible to have multiple layers in a TopoJSON file under "objects", so this provider will pull together multiple TopoJSON responses and generate a single file for efficiency in download.

To do’s:
* [x] Confirm with @mbostock that this is actually a correct interpretation of TopoJSON.
* [x] Ensure that transformations in a TopoJSON tile are compatible (determined solely by tile geometry).
* [x] Decide when to test for valid input: at configuration parse or request time?
* [x] Perform the actual combination.