Implementation for setting max volume (#336)

* Added support for creating a max volume

* Fixed db function calls

* Fixed issue with max volume always adjusting volume

configuration.example.ini

@@ -111,6 +111,9 @@ port = 64738
 #    and reload it the next time it start. It requires save_music_library to be True to function.
 #save_playlist = True
 
+# 'max_volume': Maximum volume able to be set by users. 0.0 - 1.0
+#max_volume = 0.8
+
 # 'max_track_playlist': Maximum track played when a playlist is added.
 #max_track_playlist = 20