Add JS for "Pinning" headings/dfns, like JS spec

[tabatkins]

In the JS spec, when you hover a heading you get a popup with a few options, one of which is "Pin". Clicking that puts the heading in a special new "Pins" section of the ToC, so you can easily flip back and forth between several relevant sections without having to open multiple windows. This would be cool to add into Bikeshed.