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

vendor/github.com/gomarkdown/markdown/parser/callout.go

@@ -0,0 +1,29 @@
+package parser
+
+import (
+	"bytes"
+	"strconv"
+)
+
+// IsCallout detects a callout in the following format: <<N>> Where N is a integer > 0.
+func IsCallout(data []byte) (id []byte, consumed int) {
+	if !bytes.HasPrefix(data, []byte("<<")) {
+		return nil, 0
+	}
+	start := 2
+	end := bytes.Index(data[start:], []byte(">>"))
+	if end < 0 {
+		return nil, 0
+	}
+
+	b := data[start : start+end]
+	b = bytes.TrimSpace(b)
+	i, err := strconv.Atoi(string(b))
+	if err != nil {
+		return nil, 0
+	}
+	if i <= 0 {
+		return nil, 0
+	}
+	return b, start + end + 2 // 2 for >>
+}