Make preview windows scrollable

Close #669

You can use your mouse or binadble preview-up and preview-down actions
to scroll the content of the preview window.

    fzf --preview 'highlight -O ansi {}' --bind alt-j:preview-down,alt-k:preview-up

src/options.go

@@ -663,6 +663,10 @@ func parseKeymap(keymap map[int]actionType, execmap map[int]string, str string)
 			keymap[key] = actTogglePreview
 		case "toggle-sort":
 			keymap[key] = actToggleSort
+		case "preview-up":
+			keymap[key] = actPreviewUp
+		case "preview-down":
+			keymap[key] = actPreviewDown
 		default:
 			if isExecuteAction(actLower) {
 				var offset int