Sprockets.mixins.cors

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- fix awaiting the returned coroutine if the handler's ``prepare`` function is async

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- minor packaging changes

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- initial implementation including:

- :class:`sprockets.mixins.cors.CORSMixin`
- :class:`sprockets.mixins.cors.CORSSettings`
- Support for the following pre-flight request headers:

- :mailheader:`Origin`
- :mailheader:`Access-Control-Request-Method`
- :mailheader:`Access-Control-Request-Headers`

- Support for the following pre-flight response headers:

- :mailheader:`Access-Control-Allow-Origin`
- :mailheader:`Access-Control-Allow-Methods`
- :mailheader:`Access-Control-Allow-Credentials`
- :mailheader:`Access-Control-Allow-Headers`

- Support for the following in-line response headers:

- :mailheader:`Access-Control-Allow-Origin`
- :mailheader:`Access-Control-Allow-Credentials`

.. _1.0.1: https://github.com/sprockets/sprockets.mixins.cors/compare/1.0.0...1.0.1
.. _0.1.1: https://github.com/sprockets/sprockets.mixins.cors/compare/0.1.0...0.1.1
.. _0.1.0: https://github.com/sprockets/sprockets.mixins.cors/compare/0.0.0...0.1.0