Generate the `secret` parameter automatically for new Symfony projects

As per #77 I've added generating secret key and putting it into `app/config/parameters.yml`.

I've didn't throw any exception in case file would't be writable, as I believe such situation is not possible?

If this should be done in a different way (or with better error handling) please let me know.
@Pierstoval your comments are very valid, but the chance that `ThisTokenIsNotSoSecretChangeIt ` gets modified is very low. And when we support other editions, we may need to make other changes too, so in my opinion there is no need to forward-think now. That's why I really like the simple solution proposed by @jzawadzki.