Proposal: reverse geocoder

[nucccc]

I saw #2 and this is one of the few local geocoding libraries, so I had the need to implement a reverse geocoder based on it. I'm proposing a way to execute reverse geocoding from the geo_data variable inside Geocode.

Root idea is to fit a KDTree (imported from scikit-learn) on the longitudes and latitudes, so that a new latitude and longitude pair can be queried, in order to obtain the index of the nearest location.

An example of use of the branch:

from geocode.geocode import Geocode
gc = Geocode()
gc.load()

rgc = gc.build_reverse_geocoder()
rgc.nearest_geodata(11.20, 43.75)

# ['Scandicci', 'Scandicci', 'IT', 11.18794, 43.75423, '3166988', 'place', 44951]

basically a class ReverseGeocoder is returned by Geocode and that class holds the kdtree to query the nearest points.

I don't know if the PR fits your idea of the project.
In case also the way in which it's implemented or the function names could be modified. Docs could be added to this PR if it makes sense.

Let me know if it is of interest for you!