Add disallowedClasses alias to disallowedNamespaces

[spaze]

An alias to make disallowing classes more straightforward.

You can disallow a namespace or a class like:

    disallowedNamespaces:
        -
            namespace: 'Symfony\Component\HttpFoundation\RequestStack'

or like:

    disallowedClasses:
        -
            class: 'Symfony\Component\HttpFoundation\RequestStack'

Close #112

[ruudk]

This is a great improvement! 💙