Support custom choropleth border opacity

[akacarlyann]

Separate choropleth border opacity (line_opacity) from main layer opacity. Allows user to create ChoroplethViz without any fill to the polygons.

Example:

viz = ChoroplethViz('https://raw.githubusercontent.com/mapbox/mapboxgl-jupyter/master/examples/data/us-states.geojson', 
                    access_token=token,
                    color_stops=[],
                    line_stroke='-',
                    line_color='white',
                    line_width=1,
                    opacity=0,
                    line_opacity=1,
                    center=(-96, 37.8),
                    style='mapbox://styles/mapbox/dark-v9?optimize=true')
viz.show()

Closes #156

[ryanbaumann]

Looks great, thanks for the quick feature add @akacarlyann.