Add option to hide iframe scrollbars

[nbremer]

Using a background visualization that is set to use the entire window creates scrollbars in the iframe for me because it's just a little too wide and high then, which I don't want to see. (Maybe it comes from using the external plugin as well?) Could there perhaps be an option that sets the scrolling="no" attribute on the iframe?

(I've now implemented a simple data-scroll extra option in the local copy I have of the reveald3.js code, which works, but I thought it might be a nice addition generally :) )

[gcalmettes]

Hey @nbremer!

Thanks for the suggestion! A new data-scroll optional attribute has been added (See merged PR #13). It is set to yes by default.

I have updated the documentation with a new advanced configuration section, and I have added a slide in the demo deck showing the data-scroll='no' in action.

Let me know if you need any other options or things that should be improved.