Added a list-based tags feature similar to org-mode.

Syntax Example:
	[_] 0% Heading [tag] [tag] [tag]
	    [_] 0% Heading [tag] [tag] [tag]
Commands:
	,,ct	set tag to next tag in current tag list
	,,cT	set tag to first tag in next tag list
Examples in vimoutliner/plugin/votl_checkbox.otl