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

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

@@ -0,0 +1,20 @@
+package parser
+
+// isEscape returns true if byte i is prefixed by an odd number of backslahses.
+func isEscape(data []byte, i int) bool {
+	if i == 0 {
+		return false
+	}
+	if i == 1 {
+		return data[0] == '\\'
+	}
+	j := i - 1
+	for ; j >= 0; j-- {
+		if data[j] != '\\' {
+			break
+		}
+	}
+	j++
+	// odd number of backslahes means escape
+	return (i-j)%2 != 0
+}