Support for full width iframe

Hello, Thank you for the awesome theme.

Is there a way to add iframes with full width?

Current implementation:

```html
<iframe width="640" height="550" src="https://user-images.githubusercontent.com/1xxxxx.mp4" frameborder="0" allowfullscreen></iframe>
```

This works on desktop but it breaks in the mobile due to the width.  I tried using `100%` but it didn't help.
Any suggestions?
