Rewrite generator process

Simplify process to more easily contribute
* less computing variable before write file
* use condition in template

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## How it works && How to contribute

* Create a `appName` based on third argument or folder name
* Prompt `yosay`
* Check if `.yo-rc.json` exist
* Ask questions
* Save response in `.yo-rc.json`
* Format property selected to create the associate files (see `app/src/format.js`)
* Write files based on this property and list of files (see `app/files.json`)
* Install dependencies (NPM then Bower)