settings: add volume_change_step configuration option

doc/config

@@ -301,6 +301,8 @@
 #
 #seek_time = "1"
 #
+#volume_change_step = "1"
+#
 #autocenter_mode = "no"
 #
 #centered_cursor = "no"

doc/ncmpcpp.1

@@ -108,6 +108,9 @@ If you use encoding other than utf8, set it in order to handle utf8 encoded stri
 .B seek_time = SECONDS
 Base seek time to begin with.
 .TP 
+.B volume_change_step = NUMBER
+Number of percents volume has to be increased/decreased by in volume_up/volume_down.
+.TP 
 .B playlist_disable_highlight_delay = SECONDS
 Delay for highlighting playlist since the last key was pressed. If set to 0, highlighting never fades away.
 .TP