Allow bounds to be a spatial object

[LDalby]

The get_inat_obs function accepts a bounds argument where the user can specify a bounding box within which to search for observations.

I wonder if it would be possible to allow this to be a spatial object, say a simple feature (sf)? I guess that you often have some kind of spatial object (country or state boundary for example) that you'd use for visualization after having downloaded the observations.

The downside would be that it adds sf as a dependency, but on the other hand if sf were imported, then inat_map could be modified to use geom_sf.

Just a suggestion, please feel free to close this issue if you think it is a bad idea. I'm happy to take a shot at a PR implementing it if you'd like.