settigs: add support for custom visualization color

doc/config

@@ -428,6 +428,8 @@
 #
 #active_column_color = "red"
 #
+#visualizer_color = "yellow"
+#
 #window_border_color = "green"
 #
 #active_window_border = "red"

doc/ncmpcpp.1

@@ -333,6 +333,9 @@ Color of separators used in alternative user interface.
 .B active_column_color = COLOR
 Color of active column's highlight.
 .TP 
+.B visualizer_color = COLOR
+Color of visualization.
+.TP 
 .B active_window_border = COLOR
 Color of active window's border.
 .TP