wrote code to toggle ins and dels (closes #5)

Best of both worlds... show/hide diff dynamically. Remembers your choice. 

Please note the following code changes: 

* This fixes the markup to only use <ins>s and <del>s. Please only use
those, even in block sections (filed a bug on HTML5 tidy so add support
for using ins and del around block level elements, as is allowed by
HTML5).
* tidied up CSS, combined duplicate rules.
* The document now allows the reader to check a box to see the changes,
and hides them by default.
* Added localStorage support to remember reader's choice.