geom_livemap: param "location" should be easier to use with geocoder object.

Now, to use the "location" parameter I have to transform geocoder object into a pair [lon, lat] (or to a dataframe with lon/lat columns):
```
gcoder_Beijing = geocode_cities("Beijing")
point = gcoder_Beijing.get_centroids().loc[0].geometry

(ggplot() +
geom_livemap(location=[point.x, point.y], zoom=8) +
geom_polygon(map=gcoder_Beijing, fill="white") +
# geom_polygon(map=city_blocks_sample, fill="red")
)
```

I would like to just write:
```
gcoder_Beijing = geocode_cities("Beijing")
(ggplot() +
geom_livemap(location=gcoder_Beijing, zoom=8) +
geom_polygon(map=gcoder_Beijing, fill="white") +
# geom_polygon(map=city_blocks_sample, fill="red")
)
```

The "location" parameter should also accept "Point" from shapely package.