Create updated provider for Facebook

[QWp6t]

We discussed in #9 to suggest the Facebook SDK in Composer and to create a Provider for it.

I think instead of that, we should keep it simple to avoid dependencies. This is also because the Facebook SDK seems to be undecided on SemVer and could unpredictably introduce breaking changes.

I did in fact write a Provider for FacebookSDK , but I think I'm going to get rid of it and use the Graph API manually. We don't need the entire SDK for just retrieving a list of posts. API calls to /{user-id}/feed and /{page-id}/feed both return an array of posts, so we don't have to distinguish the two. In other words, $settings['resource'] could be either a {user-id} or a {page-id}.