Add support for 'full' GeoName databases

[ghost]

GeoNames databases include some countries with their full postcode, all of which have the pattern "_full.csv" in their name. This commit allows users to take advantage of these databases by adding '_full' to the country:

> import pgeocode
> nomi = pgeocode.Nominatim('gb_full')
> nomi.query_postal_code('BS8 1QU')
postal_code                   BS8 1QU
country_code                       GB
place_name                    Bristol
state_name                    England
state_code                        ENG
county_name                   Bristol
county_code                       NaN
community_name    City of Bristol (B)
community_code              E06000023
latitude                      51.4571
longitude                     -2.6073
accuracy                            6
Name: 0, dtype: object

[paddyroddy]

This is a very useful feature 🙏

[cimadure]

Wonderfull !! Many thanks

[rennyS]

im unable to use a predownloaded dataset for this, I cant access the URL for geonames website because of my work proxy ( wanting to test some things for proof of concept )