Add optional folium_map parameter to plot_routes_folium.

[vbmendes]

Closes #199.

This PR adds the folium_map parameter to the plot_routes_folium function.

For this, I refactored the plot_route_folium and broke it into 3 helper functions:

1. result_dict_to_coords to convert a result from CompassApp into a sequence of coordinates in the format (lat, lng).
2. _create_empty_folium_map
3. plot_coords_folium to plot the coordinates in a folium Map.

This way, I can reuse these three pieces of code in the plot_routes_folium function.

[nreinicke]

This looks great, thanks for adding this in!