Invoke-release

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No release version exits nicely instead of erroring.

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- Improved the main documentation.
- Added flake8 ignore instructions.
- Changed the version and changelog writers to not strip whitespace on the beginning of lines.

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- Added support for rolling back between commit and push stages when executing release.
- Added support for executing rollback_release.
- Improved output formatting.
- Included color support for different output message types.
- Added check to ensure that new version number is greater than existing version number during release.

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- Fixed a bug in ``python_directory`` customization.

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- Added missing install requirement.

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- Added a version command to the available commands.
- Made significant improvements to documentation.