Introduce gopkg.in for go module

This change introduces gopkg.in to allow everyone to use multus
code as library. Currently multus-cni uses version 'v3', hence
it hits go modules limitation, as #504 describes.

This change fix #504 with gopkg.in.

vendor/golang.org/x/net/html/render.go

@@ -134,6 +134,9 @@ func render1(w writer, n *Node) error {
 			}
 		}
 		return w.WriteByte('>')
+	case RawNode:
+		_, err := w.WriteString(n.Data)
+		return err
 	default:
 		return errors.New("html: unknown node type")
 	}