Add support for slashes in hook IDs

When matching variables in routes, gorilla/mux uses a default pattern of
"[^/]+", thereby prohibiting slashes in variable matching.  Override the
default pattern to remove this restriction.

See https://github.com/gorilla/mux/blob/v1.8.0/regexp.go#L50

Fixes #421

webhook.go

@@ -262,7 +262,7 @@ func main() {
 	// Clean up input
 	*httpMethods = strings.ToUpper(strings.ReplaceAll(*httpMethods, " ", ""))
 
-	hooksURL := makeURL(hooksURLPrefix)
+	hooksURL := makeRoutePattern(hooksURLPrefix)
 
 	r.HandleFunc("/", func(w http.ResponseWriter, req *http.Request) {
 		fmt.Fprint(w, "OK")
@@ -278,7 +278,7 @@ func main() {
 
 	// Serve HTTP
 	if !*secure {
-		log.Printf("serving hooks on http://%s%s", addr, hooksURL)
+		log.Printf("serving hooks on http://%s%s", addr, makeHumanPattern(hooksURLPrefix))
 		log.Print(svr.Serve(ln))
 
 		return
@@ -293,7 +293,7 @@ func main() {
 	}
 	svr.TLSNextProto = make(map[string]func(*http.Server, *tls.Conn, http.Handler)) // disable http/2
 
-	log.Printf("serving hooks on https://%s%s", addr, hooksURL)
+	log.Printf("serving hooks on https://%s%s", addr, makeHumanPattern(hooksURLPrefix))
 	log.Print(svr.ServeTLS(ln, *cert, *key))
 }
 
@@ -763,10 +763,21 @@ func valuesToMap(values map[string][]string) map[string]interface{} {
 	return ret
 }
 
-// makeURL builds a hook URL with or without a prefix.
-func makeURL(prefix *string) string {
-	if prefix == nil || *prefix == "" {
-		return "/{id}"
-	}
-	return "/" + *prefix + "/{id}"
+// makeRoutePattern builds a pattern matching URL for the mux.
+func makeRoutePattern(prefix *string) string {
+	return makeBaseURL(prefix) + "/{id:.*}"
+}
+
+// makeHumanPattern builds a human-friendly URL for display.
+func makeHumanPattern(prefix *string) string {
+	return makeBaseURL(prefix) + "/{id}"
+}
+
+// makeBaseURL creates the base URL before any mux pattern matching.
+func makeBaseURL(prefix *string) string {
+	if prefix == nil || *prefix == "" {
+		return ""
+	}
+
+	return "/" + *prefix
 }