Added support for common sites (Youtube)

Because a specification of my project, I need to receive a Youtube Channel link (e.g: `https://www.youtube.com/channel/UCs_n045yHUiC-CR2s8AjIwg`) and find the rss feed from this channel. I was using feedbag until now, so I just edited the gem and added what I needed.

I'm making this pull request because I believe it can helps a lot of people. The old version of Feedbag can find a RSS Feed for YT links, but it was the wrong link. I checked the Documentation and the API of Youtube and now Feedbag is returning the correct link for the feed rss of a specific channel on Youtube.

Now you can do something like that:
```ruby
Feedbag.find("https://www.youtube.com/channel/UCs_n045yHUiC-CR2s8AjIwg/")
=> ["http://gdata.youtube.com/feeds/base/users/UCs_n045yHUiC-CR2s8AjIwg/uploads"]

Feedbag.find("https://www.youtube.com/user/CienciaTodoDia/videos")
=> ["http://gdata.youtube.com/feeds/base/users/CienciaTodoDia/uploads"]

Feedbag.find("https://www.youtube.com/channel/UCs_n045yHUiC-CR2s8AjIwg/videos")
=> ["http://gdata.youtube.com/feeds/base/users/UCs_n045yHUiC-CR2s8AjIwg/uploads"]
```

I hope you can accept this pull request, it really can help a lot of people in the future. Any question, please, feel free to comment.