Need review: Add support for Safari mobile

[bahung1221]

Hi there,

I want to create an PR to add support for safari mobile (iPhone). I successful request fullscreen for video element on my company project (that work perfect for iPhone), so now I want to add this feature to screenfull.js.

Following this webkit API document: WebKit JS HTMLVideoElement.

But I faced a problem, screenfull.js stick all method (except request method) to document element while all API of Webkit HTMLVideoElement above only work on video element not the document.

Synonymous we must refactor current structure of project, i did it for all methods in my fork repo, but i don't know what is better way for refactor instance properties (isFullscreen, element, isEnabled) that only check on document element

So please help to review my commit and give me some advise about it, thank you very much,
Happy Coding!

My Commit

[sindresorhus]

I'm unfortunately not interested in adding support for the WebKit video element fullscreen thing. It's too different from the rest and this package is done feature-wise.