Do not always trigger puli with composer plugin

[Taluu]

There should be a configuration option in the extra section of the composer configuration saying if we need to activate or not the composer-plugin. Otherwise, it is run on each composer install (and update, etc) if installed globally, which runs on projects that does not seem to need puli to begin with, creating puli files then.

I had to uninstall the plugin because it was really bothersome... I'll try to make a PR or something, but this could be considered as a bc break though :/