Wrap Kirby Text Video Embed iFrames With A Div

This is a refinement to the output of Kirby Text's method for embedding YouTube and Vimeo videos.

With the popularity of responsive design and to generally gain more control over the embed, I want to suggest wrapping the `iFrame` in a `div` with a `class` of `video-container`. Now you can utilize tools like [FitVids.js](https://github.com/davatron5000/FitVids.js) to make these video embeds fluid.