custom header IDs

Created by: seancolsen

This PR enables a markdown extension called attr_list which gives us the ability to override the default auto-generated ID value on each header. Insert {:#foo} after the header text to set an id of foo. Then use http://example.com/mypage#foo to link to the section of the page beginning with the header.

Note: I'm not saying we need to start creating custom IDs all over the place. In most cases the auto-generated ones are fine. But the benefit of custom ones is that, for sections we're linking to a lot, we can preserve the links even after edits to the header text.