Automatic footnotes

[Aaron Swartz]

> One counter-argument for including them in Markdown would be that if
> you don't like the way I've decided to mark them up, don't use
> Markdown's footnoting feature.

Another possibility is to not define a specific rendering in the
"spec" but have the Perl code do something reasonable while including
a hook so that downstream programmers can override it.

I think the important thing is that if Markdown is going to be a
standard syntax it should at least have a standard way of _expressing_
footnotes, even if it doesn't have a standard way of rendering them.
That way I can write a footnote the same way in my blog entry as in my
wiki. Whether the two render them the same isn't as much of a concern
to me -- as long as they work like footnotes, I'm happy.