Bootstrap

* Added a bootstrapper to register commands
* Updated the Beverage CLI tool to use this bootstrapper to register its own commands.
* Added register_custom_commands function to drinkmenu.php
* Removed --drinkmenu option from commands. drinkmenu.php file at the root of project is now mandatory.
* Updated Init command accordingly
* Added an EXTEND.md file to explain how to add functionalities to Beverage without modifying its codebase (modules, commands and listeners)
* Updated Modules.md