Add support for HTTP and SOCKS5 proxies (#127)

As mentioned in https://github.com/davegallant/vpngate/issues/126, adding support for proxies will help bypass issues when vpngate.net is not accessible directly.

This works by:

```sh
# http proxy
sudo vpngate connect --proxy "http://localhost:8080"

# socks5 proxy
sudo vpngate connect --socks5 "127.0.0.1:1080"
```

pkg/vpn/list_test.go

@@ -9,7 +9,7 @@ import (
 
 // TestGetListReal tests getting the real list of vpn servers
 func TestGetListReal(t *testing.T) {
-	_, err := GetList()
+	_, err := GetList("", "")
 
 	assert.NoError(t, err)
 }