Always link specified packages

[SanderSander]

We should have the ability to globally define a package that should be linked in every other project.

We should maintain a list of those packages globally, and link those automatically everywhere where composer install or composer upgrade is executed.

When the package is unlinked by the user, we should go through all the project where composer-link is active and remove those links.
(Depends #19 )
(With exception projects where composer-link is installed as project dependency).