added groupings by markdown headings

#4 wanted groupings by different types of events (and so do I!).  This code allows you to group things by using markdown headers.

I would like to add some more stylistic things (have the headers be fixed for x-scroll but scroll for y-scroll) and some stylistic things, but I'm not quite sure how to do that without adding jquery as a dependency (which I don't want to do!).