Add responsive image & video handling for responsive sizes.

[danieliser]

Make it optional for non responsive sizes.

[danieliser]

https://css-tricks.com/NetMag/FluidWidthVideo/Article-FluidWidthVideo.php

[fpcorso]

@danieliser Is this one still valid?

[danieliser]

@fpcorso I think so. There is a long thread in wordpress.org forums about different video players that are super tall with lots of black bar top and bottom and other such things where the players don't know how to behave in the popup anyways.

This might go a long way to making those types of content more friendly.

That said I don't want to do anything beyond the minimum here that might impact the feature set of the Video extension.