Allow adding warning to generated multipage specs

[tobie]

Now that we're using Wattsi to build spec previews and HTML diffs, it would be nice if there was an option to add a warning message (e.g. via a modal dialog) to the generated multipage specs that wattsi-server could trigger.

[domenic]

This is whatwg/html-build#97, essentially.

Edit: well, on second glance, it's another form of it; they're different, but extremely related.