Support local coordinate systems

[visr]

@Huite showed me https://github.com/Huite/projecttile last week, and also geopandas/contextily#120.

I'm guessing we're almost there already since the code is CRS-aware with the WGS84 and WebMercator types, but it would be good to have an example where we use Proj.jl and a local provider like in geopandas/contextily@fc4a0fb to make sure it all works.

[asinghvi17]

So for full CRS awareness I think we should allow arbitrary boundingboxes going in as well - the only restriction being that the structure has to be a quadtree.

Seems like we would want an AbstractTile interface then though.