Add option to enable responsive video embeds

[cjg89]

See UCF/Today-Child-Theme#19 as an example. A snippet like this would help ensure that video embeds (YouTube, Vimeo, etc) are always responsive when included in page/post content.

However, since some sites may already be accommodating for this by adding wrapper markup manually, we'll need to either:

a) add an option, disabled by default, that activates the responsive embed wrapper markup, or

b) adds some additional logic to the_content (or elsewhere?) that detects duplicate <div class="embed-responsive ..."> wrappers and consolidates them.

[cjg89]

Added option to enable responsive oEmbed and video content in v0.4.0.