Add a default die-on value to the spec metadata

[jyasskin]

As mentioned in whatwg/meta#286 (comment), it'd be great for specs to be able to include their default --die-on value in their metadata. The fact that a spec is warning-clean is a property of the specification, so every build shouldn't have to remember to pass that argument.

[tabatkins]

Yeah, that sounds reasonable. It's a little bit of a layering violation since you can get errors before I even successfully parse the metadata, but that's fine, I'll just have to check if any errors have been emitted after metadata parsing is complete.

[inexorabletash]

It looks like maybe this happened in 82444e6

... but this issue isn't closed, and the docs weren't updated. Is there more to be done?

[tabatkins]

I need to fix up the docs, is all.