Allow for other coordinate systems (e.g. UTM)

I would like to be able to use UTM coordinates for my axes in ggmap. I currently do this by getting my background with `get_map()`, converting the bounding box attributes via `rgdal::spTransform`, and using `ggplot() + inset_raster() + ... + coord_fixed()`. For small map areas, UTM are more understandable units because they are in meters. It would probably be helpful to be able to convert to meters/km and/or set the axes to start at zero at the origin.