div & span support

[John Fraser]

I don't think Markdown needs its own syntax for div elements; however
you write it, a div is only going to make sense to someone who knows
what a div is.  And those people can already read and write `<div
id="title">` easily enough -- without learning new syntax.

If a reader doesn't happen to know what a div is, they can at least
tell that there's something called a div, and it's got the word title
in it, and it seems to contain something that looks like it might be a
title.  I don't think any syntactic changes you can make are going to
improve much on that.  On the contrary: they'll just take the word
"div" away, leaving people with nothing to google if they don't know
what they're looking at.

I really feel like the best approach is to get Markdown working inside
normal HTML divs, just like it works inside spans right now.

-John