allow for custom element tag

Some versions of iOS have unusual behavior when in comes to resizing iframes. They disallow doing it at all, css width and height is being ignored, and also iframe is as wide as it's content - meaning that if content exceeds iframe size, iframe will be widened.
Easiest way I've found to deal with this problem is to use <object> tag instead of <iframe>. It essentially behaves the same, but doesn't have resizing problems.
Hence the pull request, maybe let's use iframes by default but allow user to use objects if needed?