Processor

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* Function ``run_pipline`` was simplified and now accepts only one source and one ouput.
To implement more complex pipelines, use ``sources.mix`` and ``outputs.fanout`` helpers.

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* Added a `web.hook`_ source.
* Now `source` could be not only a iterable object, but any function which returns values.

.. _web.hook: https://python-processor.readthedocs.org/en/latest/sources.htmlweb-hook

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Fixed error in ``import-or-error`` macro, which prevented from using 3-party libraries.

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Most 3-party libraries are optional now. If you want to use
some extension which requires external library, it will issue
an error and call ``sys.exit(1)`` until you satisfy this
requirement.

This should make life easier for thouse, who does not want
to use ``rss`` output which requires ``feedgen`` which requires
``lxml`` which is hard to build because it is C extension.

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* First release on PyPI.