Merge pull request 'oopsie: add function that i forgot to commit' (#3) from ctucx/webmusic:master into master, fixes #2

Reviewed-on: https://git.zotan.services/zotan/webmusic/pulls/3

webmusic.js

@@ -142,6 +142,14 @@ function initState() {
     total = queue.length;
 }
 
+function previousTrack() {
+    if (index-- === 0)
+        index = total-1;
+    if (continuelist) {
+        playSong(queue[index])
+    }
+}
+
 function nextTrack() {
     if (++index === total)
         index = 0;