fix width/height issue for layer_static_mapbox

[elipousson]

Hopefully, the third time is the charm and this fully addresses #17. Here are the changes:

• refactor: add bbox_to_center helper function
• fix: correct width/height for layer_static_mapbox
• refactor: add set_static_map_dims and set_static_map_overlay helper functions
• feat: add style_url parameter to get_style, addMapboxTiles, and other functions
• docs: Replace "Your Mapbox access token" with "A Mapbox access token"

Let me know if you don't want to support the style_url parameter or prefer the "Your Mapbox access token" phrasing and I can reverse those changes. They aren't essential to getting layer_static_mapbox to work although I think the style_url parameter should be a nice convenience.

[walkerke]

I'm good with those changes and I think it's nice to give users flexibility with the style URL. Thanks again!