add vendoring

vendor/github.com/yuin/goldmark/extension/ast/strikethrough.go

@@ -0,0 +1,29 @@
+// Package ast defines AST nodes that represents extension's elements
+package ast
+
+import (
+	gast "github.com/yuin/goldmark/ast"
+)
+
+// A Strikethrough struct represents a strikethrough of GFM text.
+type Strikethrough struct {
+	gast.BaseInline
+}
+
+// Dump implements Node.Dump.
+func (n *Strikethrough) Dump(source []byte, level int) {
+	gast.DumpHelper(n, source, level, nil, nil)
+}
+
+// KindStrikethrough is a NodeKind of the Strikethrough node.
+var KindStrikethrough = gast.NewNodeKind("Strikethrough")
+
+// Kind implements Node.Kind.
+func (n *Strikethrough) Kind() gast.NodeKind {
+	return KindStrikethrough
+}
+
+// NewStrikethrough returns a new Strikethrough node.
+func NewStrikethrough() *Strikethrough {
+	return &Strikethrough{}
+}