Adds support for markdown footnotes and linking to headings

[dwwoelfel]

To use footnotes in markdown, just do this:

Here's a simple footnote,[^1] and here's a longer one.[^bignote]

[^1]: This is the first footnote.

[^bignote]: Here's one with multiple paragraphs and code.

    Indent paragraphs to include them in the footnote.

    `{ my code }`

    Add as many paragraphs as you like.

In your last post, you'll want to make a change like:

- Notice that the wires carry a charge, but we choose to interpret _meaning_ in the charge. “high charge” means 1, and “low charge” means 0. Nothing changes in the machine, this is just something we decided as humans (1).
+ Notice that the wires carry a charge, but we choose to interpret _meaning_ in the charge. “high charge” means 1, and “low charge” means 0. Nothing changes in the machine, this is just something we decided as humans[^1].
...
- (1) I say this pretty lightly, but treating the charge on circuits as code was a pretty [phenomenal innovation](https://en.wikipedia.org/wiki/Claude_Shannon#Logic_circuits). I could only imagine what it might have been like, when Shannon showed a full-adder.
+ [^1]: I say this pretty lightly, but treating the charge on circuits as code was a pretty [phenomenal innovation](https://en.wikipedia.org/wiki/Claude_Shannon#Logic_circuits). I could only imagine what it might have been like, when Shannon showed a full-adder.

You can use whatever identifier you want (e.g. 1 or bignote above), but probably best to keep with numbered footnotes because it looks a bit nicer when you view it on GitHub.

This also adds support for linking directly to headlines. You would link to the 7: Simulate Enabler headline like so:

This is a [link to simulate enabler](#7-simulate-enabler)

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[stopachka]

aweesome!