allow some way to pass more options to global composer.json

[ghost]

I was testing this cachetool package (to clear php opcache), but realized that to make it use my forked version, Ihad to manually edit the composer.json file to add my repository.

BTW: It's nice to have a way to clear the opcache without restarting php-fpm or anything like that: https://github.com/gordalina/cachetool