Add config setting for allowfullscreen iframe attribute

[xemlock]

Related to #37.

Need something similar to HTML.FlashAllowFullScreen.

[bytestream]

Ref: https://fullscreen.spec.whatwg.org/#security-and-privacy-considerations

[xemlock]

@bytestream It turns out the additional config option is actually not needed. Iframe is defined in an Iframe module, which is marked as unsafe - it won't be enabled unless HTML.Trusted is explicitly set to true.

The HTML.FlashAllowFullScreen setting is used in SafeObject module, because the module defines <object> tags in safe mode - and only allows carefully filtered subset of params. Unsafe Object module has no such restrictions.

So, the only change required is to define HTML5_Iframe module which inherits from Iframe module, and defines allowfullscreen attribute.

[xemlock]

Ok, apparently I was wrong 😅. Iframe can be included not only by setting HTML.Trusted to true, but also by HTML.SafeIframe. In such case the switch (e.g. HTML.IframeAllowFullscreen) is required.