Make fps of the visualizer configurable and set it to 60 by default

doc/config

@@ -84,6 +84,8 @@
 ##
 #visualizer_type = wave
 #
+#visualizer_fps = 60
+#
 #visualizer_autoscale = no
 #
 #visualizer_look = ●▮

doc/ncmpcpp.1

@@ -100,6 +100,9 @@ Defines visualizer's look (string has to be exactly 2 characters long: first one
 .B visualizer_color = COLORS
 Comma separated list of colors to be used in music visualization.
 .TP
+.B visualizer_fps = FPS
+The amount of frames per second for the visualizer.
+.TP
 .B visualizer_autoscale = yes/no
 Automatically scale visualizer size.
 .TP