Initial support for embedded media within liveupdates.

:eyeglasses: @spladug @chromakode

Note: Companion PR into reddit proper required first for config support for sets!

You'll also want to set `liveupdate_embeddable_domains` in your development.update. I've got mine set to: 
```
liveupdate_embeddable_domains = youtube.com, youtu.be, twitter.com, t.co, imgur.com, flickr.com, flic.kr
```

OK guys, here's a first cut of what embedded media is looking like. There's still a number of open questions but I wanted to get this to you before I left for the week.

Some open issues:
* You won't be able to use twitter embeds yet, as that requires the TwitterScraper that I haven't PR'ed yet. I've got it written and it works, biggest decision is whether we should make it only work for live or make it used on reddit proper as well. Any thoughts on that? Youtube should work just fine though, you can play with that if you're testing.
* Heights of iframes are going to be a problem with twitter (their heights are dynamic), so we're going to need to need to set the height of the iframe dynamically ourselves (probably using postMessage)
* There's a bunch of stuff that I'm still not sure on, things like how to best handle individual embeds. Right now it's a multi-value list just on the update itself, which means I'm using raw indexes to get at them in the URI. That seems kinda gross to me from a relational standpoint but I'm not sure if it's "normal" schemaless wise.

I'd love any feedback on all of this, especially if I'm doing things weirdly in terms of the way y'all do things or how to use plugins! I think I'm getting my footing a bit more but there's a lot to cover. ;) Please be harsh!