Add optional MultiPolygon output to union

[dnomadb]

Here https://github.com/mapbox/supermercado/blob/master/supermercado/uniontiles.py#L29-L35 we spit out each separate feature as a single feature; let's add an option to output to MultiPolygon

cc @vincentsarago

[dnomadb]

Here's an inline converter in the interim: https://gist.github.com/dnomadb/ea8b609b252097d357744b3ef4f13a96