Ffmpeg-normalize

* Do not print stream warning when there is only one stream.

* Remove previous dist versions before release.

* Warn when duration cannot be read, fixes 105.

* Update README.

minor improvements in the description

* Add output to unit test failures.

* Fix input label for audio stream.

* Add pre-and post-filter hooks, fixes 67.

This allows users to specify filters to be run before or after the actual
normalization call, using regular ffmpeg syntax.
Only applies to audio.

* Document audiostream class.

* Warn when file is too short, fixes 87.

* Update release method to twine.

* Handle progress output from ffmpeg, fixes 10.

* Merge pull request 99 from Nottt/patch-1.

fix -cn description

* Fix -cn description.

* Add nicer headers for options in README.

* Add version file in release script before committing.

* Add option to keep original audio, fixes 83.

* Add pypi badge.

* Allow release script to add changelog for future version; upload to pypi.