Add support for third-party extractors

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

    # in mypackage/setup.py
    setup(
        ...
        entry_points={
            'youtube_dl.extractors': [
                'myextractor = mypackage.extractor.MyExtractorIE',
            ],
        },
    )
make sure it works if pkg_resources is not available
Explain setuptools entry points in the README