Github OAuth App - Federated Login

Do you want to request a feature or report a bug?
Feature

What is the current behavior?
Federated login for Google, Amazon and Facebook but not Github.

What is the expected behavior?
the Authenticator HOC will include a "Sign-in with Github" button.

This issue is identical to aws-amplify/amplify-cli#995 except for [github as the identity provider](https://developer.github.com/apps/building-oauth-apps/).
[Keycloak has github as an identity provider](https://www.keycloak.org/docs/2.5/server_admin/topics/identity-broker/social/github.html) so i have to use that in my amplify app in addition to Cognito.