Add offline mode which disables sig check for local debugging

This release adds two new options to app.config:

• DONT_VALIDATE_SIGNATURE disables validating the signature on incoming POST requests
• DONT_REGISTER_WITH_DISCORD disables registering slash commands with the Discord API

These can be used to debug a bot locally using Postman, cURL, or a similar tool. Read the documentation here for more information.

Thank you @akiller for submitting and documenting this patch! (#13)