Support requirement extras in mapping file

Per #61, it's occasionally useful to be able to use an `extras` requirement (e.g. `requests[security]` or `scikit-learn[alldeps]`). This can be accomplished using [`Requirement` objects](https://setuptools.readthedocs.io/en/latest/pkg_resources.html#requirement-objects) and the `Requirement.parse` method to extract the name prior to performing a search against the package index API and to add it back during the serialization step.