GitHub two-factor TOTP generation

This adds the `gh token` command for generating a Time-based One-Time Password (TOTP). This is helpful for when you create a standalone user for the Lita bot to administer your organization. Without this functionality, one person would need to be responsible for the token.

To have this command function you need to add the following option to your config:

`config.handlers.github.totp_secrent = '<secret_key>'`

You can get this secret by selecting 'enter key manually' when setting up two-factor using an app on your phone. If you lost the secret you can obtain a new one by reconfiguring two-factor authentication.