Support customizable variables in templates

[YassineElbouchaibi]

Hey, this is a feature request and I would possibly want to work on it as I'm looking for simple stuff to do on my free time.

Basically what I'm suggesting is adding to the cli the ability to ask some questions defined by the template maker to customize the template. The questions would be defined in a json that would live inside the template's folder.

Here are few examples :

• A firebase template could ask for apiKey, authDomain, databaseURL, ...
• Another template could ask for a primaryColor, secondaryColor, ...
• Many more applications

No json would mean no customization.

Let me know if you think something like this would be useful