(#14) Added support for Facebook federation

[adrianhall]

Issue #, if available:

14

Description of changes:

• Added support for generating FacebookSignin section in awsconfiguration.json
• Added support for facebook_app_id in the aws-exports.js (not used by Amplify)
• Added test example in the example serverless.yml file

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.