Need to poll Can I Use for new features that need annotations

[tabatkins]

As described in whatwg/dom#69, a problem with the current Can I Use annotation feature is that you have to be aware of all the Can I Use features that are available, and need to regularly check to see if any new ones show up that talk about a feature from one of your specs.

I should add a metadata that lets you specify the URL of your spec, and match it against the spec URLs recorded for each of the Can I Use features; if there are any hits, and you're not already specifying that annotation somewhere in your spec, it should log a warning.

This obviously relies on the Can I Use features having good URLs. I should also surface that information to make it easier to tell what the URLs are, so people can PR Can I Use to get things fixed.

[tabatkins]

Done, via the Can I Use URL metadata.