Support custom tile layers

[gmaclennan]

We should support custom tile layers and have initial options that include both satellite tiles and vector tiles. We should store background tile options on state so that we can support dynamically adding new layers and sharing config that has different tilesets.

To do this with the flexibility to accept any style layer we will need to fix #51 first.

[okdistribute]

Now you can add a layer control to the map which sort of helps get there, here's an example for reference:
https://github.com/digidem/mapfilter-desktop/tree/mapbox-gl-layers