Add Gitlab, bind methods, and update CLI

[falcucci]

adds the Gitlab class to the src/index.js file, binds the isEnabled method to the Slack class in src/Slack.js, and updates the CLI to use the Github class instead of the Gitlab class in src/cli.js.

In src/index.js, the Gitlab class is added to the module.exports object. This allows the Gitlab class to be imported and used in other files.

In src/Slack.js, the isEnabled method is bound to the Slack class. This allows the isEnabled method to be used as a class method, rather than a standalone function.

Finally, in src/cli.js, the Github class is imported instead of the Gitlab class. This allows the CLI to use the Github class instead of the Gitlab class.