A brand new version of MentionToSlack app

[Laurenfrost]

There are too much restrictions in current version. Especially the lists of GitHub User ID to Slack User ID, are stored separately in different branches of different repos. That means when you want to add a new user, you have to update all the list.yaml for every single branch in every repo, which is almost impossible to do so. On the other hands, Slack Incoming Web-Hook, the API that MentionToSlack currently depends on, is going to be deprecated soon. It's time to update the whole framework, but in a new Repo.