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
Minor edit of comments.
Changed the math key mappings.
<localleader>==		compute current heading up to its parent
<localleader>=t		compute current tree (down then up)
<localleader>=d		compute entire document
<localleader>=h		hide math (set conceallevel=1)
<localleader>=H		show math (set conceallevel=0)
There should be no change of the vim default colorscheme!

Fixes #82

Change-Id: Id8ea8f450854e76fd277e0dd9bdf39b52378e1a4