#97 Added prompt for missing magento root dir

I just added the requested feature
- If no extra.magento-root-dir ist defined the user is prompted to enter a directory
- The entered value gets inserted into the composer.json file
- If the root folder does not exist within the composer.json dir, directories are created recursively until the reqested path is valid

There are no unit tests yet and it's basically just a first test. 
I'll gladly address any issues you report :)