require-vpn per sobolevn

Configuration:

- `VPN_IP` - your VPN IP, which is expected
- `BLOCKED_HOSTS` - a new-line separated list with hostnames of blocked resources, default: `['example.com']`
- `IP_CHECKER_URL` - a URL to any service that will receive a `GET` request and return your API in plain text, default: `http://icanhazip.com`
- `PROXY_URI` - a URI for your proxy server to where a redirect will be made, see [here](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/proxy/ProxyInfo) what formats are supported, default: `http://localhost:63555`