Mullvad: configurable ipv6 tunneling (#253)

- Disabled by default
- Wiki pages updated
- Readme updated

internal/params/params.go

@@ -54,6 +54,7 @@ type Reader interface {
 	GetTargetIP() (ip net.IP, err error)
 	GetOpenVPNCipher() (cipher string, err error)
 	GetOpenVPNAuth() (auth string, err error)
+	GetOpenVPNIPv6() (tunnel bool, err error)
 
 	// PIA getters
 	GetPortForwarding() (activated bool, err error)