Add environment variable to filter by country code

[dvdblg]

This commit adds the possibility to choose the country you want to connect to by simply setting the environment variable COUNTRY_CODE to a country code such as US, DE or CH.

[imthenachoman]

This is slick. It seems like it would be easy-ish to add more environment variables for filters. I wish NordVPN had documentation on the API.

[bubuntux]

Thanks for the contribution, this is sick