Update markdown parsing library to github.com/gomarkdown/markdown (#944)

vendor/github.com/gomarkdown/markdown/fuzz.go

@@ -0,0 +1,9 @@
+// +build gofuzz
+
+package markdown
+
+// Fuzz is to be used by https://github.com/dvyukov/go-fuzz
+func Fuzz(data []byte) int {
+	Parse(data, nil)
+	return 0
+}