Show attribution of the used provider on the map

[felixcremer]

I think, that the attribution of the tile provider should be shown as a default on the lower part of the map.
The attribution should be available as a metadata entry in the TileProvider.

[rafaqz]

Totally, Leaflet does this.

The provider options dict has html_attribution and attribution fields - but I should make wrapper functions to get these in TileProviders.jl.