Footnotes

Found myself wanting this feature as I was writing a blog post earlier.

``` markdown
Some text with a citation.{1}

***

{^1}Further explanation goes here.
```

It's an inline, not a block, element, which I thought was a lot more flexible. There's nothing preventing the user from swapping `{1}` and `{^1}` and getting the same behavior.

1. Should we prevent people from using `{^x}` in body text?
  * How?
2. Should this be a block element?