add support for third-party extractors

Other packages can register new extractors through the
`youtube_dl.extractors` setuptools entry-point.

For example:

    setup(
        ...
        entry_points={
            'youtube_dl.extractors': [
                'myextractor = mypackage.extractor.MyExtractorIE',
            ],
        },
    )
make sure it works if pkg_resources is not available