Add offset-up and offset-down

# Scrolling will behave similarly to CTRL-E and CTRL-Y of vim
  fzf --bind scroll-up:offset-up,scroll-down:offset-down \
      --bind ctrl-y:offset-up,ctrl-e:offset-down \
      --scroll-off=5

Close #3456

src/options.go

@@ -1179,6 +1179,10 @@ func parseActionList(masked string, original string, prevActions []*action, putA
 			appendAction(actTogglePreviewWrap)
 		case "toggle-sort":
 			appendAction(actToggleSort)
+		case "offset-up":
+			appendAction(actOffsetUp)
+		case "offset-down":
+			appendAction(actOffsetDown)
 		case "preview-top":
 			appendAction(actPreviewTop)
 		case "preview-bottom":