Use extras_requires in setup.py for soft dependencies

Using [`extras_requires`](https://setuptools.readthedocs.io/en/latest/setuptools.html#new-and-changed-setup-keywords) in `setup.py` might make it simpler to handle soft dependencies. This allows one to install cclib as

    $ pip install cclib  # default dependencies
    $ pip install cclib[extra_feature]  # default + dependencies for 'extra_feature'