ScripHandler method to dump assets on composer install/update

such method was originaly added by @arjenjb but disappeared after a rebase

Simple dumpAssetic() method to add such a command on composer install/update.
I just copy/paste and customize to call assetic:dump command.

Maybe I could refactor static methods for common assertions (like checking if $appDir exists) rather than duplicate code in each method. Is there any interest ?