Support for third-party Yang extensions implementation

It would be nice if yangtools parser can be initialized to use custom third-party extension implementation.
If yang-parser is used to build EffectiveModelContext from yang sources it is using extensions internally implemented in yang-parser-impl project.
I would be nice if yang-parser-api is extended, so it is possible to create an instance of yang parser, register custom extensions in it and build EffectiveModelContext containing custom defined extensions.