Add ability to run markdown on multiple separate elements.

Hi,

Firstly, thanks for creating this - I've been finding it really useful.

I had a requirement to have two distinct markdown blocks on the same page, so I made these changes to iterate over all the possible elements to apply markdown to.  Very little has changed apart from wrapping a loop around some of the original code.

I've included a demo page to show what I mean.  This requires a `0.3` bundle to be generated.

I thought I'd send a pull request in case you're interested in including the changes.

Cheers.