Add referenceWidth and referenceHeight to the configuration options

[yanntrividic]

This repo seems to be a bit dormant, but I like this project and I've been using it for some time now, so I thought it made sense to still submit an issue regarding this.

The Scaler makes it so the slides are device agnostic. Whatever the device you use, you should see the same thing. The problem I find with it is that there is no way to change the reference dimensions it sets, at least to my knowledge.

Those dimensions are currently a referenceWidth of 908px and a referenceHeight of 681px.

In most cases, it is not a problem, because you handle the CSS of your slides yourself. But when you integrate iframes loading HTML and CSS code from other websites, those values do matter to you because they define the dimensions of the content of those iframes.

It would be nice to be able to tweak them in the configuration options!