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/node.go

@@ -18,6 +18,11 @@ const (
 	ElementNode
 	CommentNode
 	DoctypeNode
+	// RawNode nodes are not returned by the parser, but can be part of the
+	// Node tree passed to func Render to insert raw HTML (without escaping).
+	// If so, this package makes no guarantee that the rendered HTML is secure
+	// (from e.g. Cross Site Scripting attacks) or well-formed.
+	RawNode
 	scopeMarkerNode
 )