add interactive prompt for folder permissions in composer postInstall

ref to #121.
Add an interactive prompt to ask user if he wants to change folder permissions.
Default to Yes, but I can change it for No. Let me know if you want any change.
You might be able to do this more elegantly using [askAndValidate()](https://github.com/composer/composer/blob/master/src/Composer/IO/IOInterface.php#L116)