Give Headings Unique IDs

We should automatically give headings an ID with their title in it.

For example, the following markdown text:

This's an Example Title!
------------------------

Should come out as:

<h2 id="thiss-an-example-title">This's an Example Title!</h2>

That way we can easily link to specific bits in the docs like a section on a longer HIG page. If needed we could always prepend "heading-" or something to make sure it's unique, and of course we'd want to drop special characters, replace spaces with dashes (or underscores), and lowercase everything as I've done above.