Add url_template and layer_options arguments to GeoJSON display class

[gnestor]

This would enable users to configure Leaflet and e.g. map points on other planets.

from IPython.display import GeoJSON

GeoJSON(data={
    "type": "Feature",
    "geometry": {
        "type": "Point",
        "coordinates": [-81.327, 296.038]
    },
    "properties": {
        "name": "Inca City"
    }
}, 
url_template="http://s3-eu-west-1.amazonaws.com/whereonmars.cartodb.net/{basemap_id}/{z}/{x}/{y}.png", 
layer_options={
    "basemap_id": "celestia_mars-shaded-16k_global",
    "attribution" : "Celestia/praesepe",
    "minZoom" : 0,
    "maxZoom" : 18,
})

[gnestor]

@Carreau @takluyver This think this is ready

[Carreau]

I've added test, docs and a line in the what's new. Let me know if that's allright.

[takluyver]

#10253

[takluyver]

g -> gj

[Carreau]

Thanks both !