Allow specifying document via an option

[Thristhart]

We use Flipsnap in an application that involves iframes. Unfortunately, mouse/touch events don't work properly in our use case, because the document that Flipsnap uses is different than the contentDocument of the iframe. We could fix that by passing a reference to the contentDocument as an option, and Flipsnap could use that instead of the top level document. I believe there are only a couple instances that matter -- in _touchStart and _touchEnd, when the eventListeners are added.

I'm posting this as an issue rather than a pull request because I want to see if you agree that this is the best way to solve the problem, but if you like I can submit a pull request as well.