add fuzzing driver and crashes found with it

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
+}

fuzz_crashes_test.go

@@ -0,0 +1,27 @@
+package markdown
+
+import "testing"
+
+// crashes found with go-fuzz
+
+func TestCrash1(t *testing.T) {
+	tests := []string{
+		": \n\n0\n00",
+		">>>0```\n\n:\n```",
+		">0```\n: \n\n0\n```",
+		">>>>0```\n\n:\n```",
+		"0\n\n:\n00",
+		">>0```\n\n:\n```",
+		"[0]:<",
+		"[[[[[[\n\t: ]]]]]]\n\n: " + "\n\n:(()",
+		">0\n>\n:\n00",
+		": : \n\n\t0\n00",
+		"0\n: : \n\n\t0\n00",
+		"0\n\n:\n00",
+		"0\n\n: [0]:<",
+		"[0]:<",
+	}
+	for _, test := range tests {
+		Parse([]byte(test), nil)
+	}
+}