bcmaps v0.18.0

CRAN release v0.18.0

• Added bc_neighbours() function to call layers containing adjacent jurisdictions.
• Ensured the geometry column in all layers is consistently named "geometry" (Thanks @boshek)
• Moving sf package to Depends to take advantage of sf print methods.
• Added bc_bbox() to get a bounding box for British Columbia (#40).
• All references to external data package now point to bcmapsdata because of this R bug (https://bugs.r-project.org/bugzilla/show_bug.cgi?id=17520)