Add api for annotations

this only returns the first available annotation for now
annotation content is returned in html format, so it can easily be
inserted in the frontend

displaying the annotation in the frontend is still a TODO

main.go

@@ -29,6 +29,7 @@ func main() {
 	r.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) { render("home", w, nil) })
 	r.HandleFunc("/search", searchHandler).Methods("GET")
 	r.HandleFunc("/{id}-lyrics", lyricsHandler)
+	r.HandleFunc("/{id}/{artist-song}/{verse}/annotations", annotationsHandler)
 	r.HandleFunc("/images/{filename}.{ext}", proxyHandler)
 	r.PathPrefix("/static/").Handler(http.StripPrefix("/static/", http.FileServer(http.Dir("static"))))
 	r.NotFoundHandler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {