Move format exports into child classes.

To only export implemented methods.
Add new TopoJSON format.
Bring topojson examples back
Add TopoJSON source & use it in example
Bring tests back
Add TopoJSONOptions literals for source & format
Make TopoJSON tests use new format
Add support for reading id & properties
Better styling in example
Merge pull request #1496 from tonio/vector-api-topojson

[vector-api] Add TopoJSON format