Namespace prefix parameter for namespace converter plugin

This PR adds the parameter *NamespacePrefix* to the LegacyNamespaceConverter Plugin.
It is useful to have this option in scenarios where legacy namespaces without vendor prefixes co-exist together with PSR-4-style namespaces (with vendor prefixes).

e.g. `Fancy_LegacyClass` can be prefixed with `My` and will end up in the same namespace together with `My\Fancy\NewClass`

Feel free to review and comment. Thanks!