make color of separators used in alternative user interface customizable

doc/config

@@ -304,6 +304,8 @@
 #
 #statusbar_color = "default"
 #
+#alternative_ui_separator_color = "black"
+#
 #active_column_color = "red"
 #
 #window_border_color = "green"

doc/ncmpcpp.1

@@ -274,6 +274,9 @@ Color of progressbar.
 .B statusbar_color = COLOR
 Color of statusbar.
 .TP 
+.B alternative_ui_separator_color = COLOR
+Color of separators used in alternative user interface.
+.TP 
 .B active_column_color = COLOR
 Color of active column's highlight.
 .TP