add lazyload optin in help
Keep only HTML output

I'm using multimarkdown for personal use only, and I only need HTML output with some tweaks, so I removed the stuff I didn't need, there's probably more code that could be deleted but it's a starting point.
Tweaks footnotes

Allow to inline footnotes in a hidden span instead of actually having the note at the bottom.

Then we can display the note content using some JS.