Add vignette for generating pretty maps for BC

Pending the release of geom_sf a theme for bcmaps would be useful.

Examples (but not exactly what we after) include:
- https://andybeger.com/2012/08/25/scale-and-north-arrow-for-maps-in-r/
- https://github.com/paleolimbot/prettymapr

Start by reviewing the issue's geom_sf context and the linked scale-and-north-arrow and prettymapr examples. Inspect the bcmaps package structure to find the appropriate vignette location. Done means a vignette explains and demonstrates a useful approach for generating pretty British Columbia maps, with the intended theme defined clearly.

Written by the indexing model from the issue text.