Add support for instagram embeds

[dawaltconley]

I'd be interested in submitting a pull request to support instagram embeds.

Those are a little trickier in that they require a script tag, which you usually don't want more than one of per-page. As part of that / allowing people more control over the output HTML I'd like to also submit a pull request exposing the function that converts urls to their embeddable versions to the user. Something like this

var embed = require('embed-video');
embed.url(/* youtu.be url */) // returns youtube.com/embed url

Though there may be better ways to handle that, and it may be useful to also return the video "type" ('youtube', 'vimeo', etc) in some cases.

Let me know if either / both would be welcome.