Add support for experimental-html-handler.

Small diff which allows library users to override the el.innerHTML assignment.
Useful when special behavior is needed.
You think this looks better?
            innerHtmlHandler(extracted.html, el, function() {
              spf.tasks.resume(key, sync);  // Resume queue after handler.
              installScripts();
            });

gjslint is really obnoxious about indentation but apparently the snippet above is a valid option. Using that instead.