Speed up mungedoc TOC by pre-compiling a regex

Brings the time to run the TOC over the docs directory from .7 seconds
to .1 seconds

cmd/mungedocs/toc.go

@@ -57,12 +57,13 @@ func buildTOC(markdown []byte) ([]byte, error) {
 	buffer.WriteString("\n")
 	scanner := bufio.NewScanner(bytes.NewReader(markdown))
 	inBlockQuotes := false
-	for scanner.Scan() {
+	blockQuoteRegex, err := regexp.Compile("^```")
-		line := scanner.Text()
-		match, err := regexp.Match("^```", []byte(line))
 	if err != nil {
 		return nil, err
 	}
+	for scanner.Scan() {
+		line := scanner.Text()
+		match := blockQuoteRegex.Match([]byte(line))
 		if match {
 			inBlockQuotes = !inBlockQuotes
 			continue