Adding proxy configuration

[andersoncarlosfs]

Manual test:

# Import Meteostat library
from meteostat import Stations

# Set a public proxy
Stations.cache_dir = './test/filtered/'
Stations.proxy = '81.255.13.197:8080'
#Stations.max_age = 0

# Get nearby weather stations
stations = Stations()
stations = stations.nearby(49.2497, -123.1193)
station = stations.fetch(1)

# Print DataFrame
print(station)