Wireguard support for Mullvad and Windscribe (#565)

- `internal/wireguard` client package with unit tests
- Implementation works with kernel space or user space if unavailable
- `WIREGUARD_PRIVATE_KEY`
- `WIREGUARD_ADDRESS`
- `WIREGUARD_PRESHARED_KEY`
- `WIREGUARD_PORT`
- `internal/netlink` package used by `internal/wireguard`

internal/netlink/route.go

@@ -0,0 +1,7 @@
+package netlink
+
+import "github.com/vishvananda/netlink"
+
+func (n *NetLink) RouteAdd(route *netlink.Route) error {
+	return netlink.RouteAdd(route)
+}