Allow dict input for Map Style

[kylebarron]

The dict should be a valid mapbox style spec object.

We might also want to have helpers for a single-layer raster-tile basemap. E.g. lonboard.basemap.RasterBasemap which takes in an xyz url. We should have an example with standard OSM tiles.

There are a bunch of default public raster tile basemaps here (referenced from here). Also see https://github.com/geopandas/xyzservices, which may be a better integration point. We should be able to integrate directly with the TileProvider class.