Make application not only nord vpn specifiec

[hapylestat]

Refactor code, to split NordAPI/OpenVPN/Application itself as self-sustained chunks of the application and let another people to write interfaces to their own VPN providers

[jbweston]

This is already quite separated. I think the only thing that depends on the NordVPN API itself is nord/api.py, which provides the Client class. Similarly, the OpenVPN part is entirely contained in nord/vpn.py.

When using the command line interface the choice of using the Nord client is currently hardcoded, but this would not be a very big change to have this configurable. The work would be writing a Client class that interacts with the VPN service of choice.

Did you have a particular one in mind?

[hapylestat]

I thinking about PureVPN as it have better countries coverage, but currently i'm still nord user (: