Invoke-release

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- Support the latest version of Invoke, which requires context arguments for tasks

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- Fixed a bug Senna discovered: If path to Vim doesn't exist, a different error is thrown.

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- Fixed issue with release tool using vi instead of vim by default and vi being linked differently in path
- Changed the default from ``vi`` to ``vim``, because it has been established that a ``vi`` command linked to a non-``vim`` executable doesn't work.
- Added the ability to specify an Invoke Release-specific editor environmental variable that doesn't conflict with other applications' use of ``$EDITOR``.
- Added exception checking to suggest using the environmental variable if an editor fails to launch.
- Improved error messages for other command error messages, because ``CalledProcessError`` never has a value for ``e.message``.

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- Made file exist checks case-sensitive, because Git is always case sensitive, even though Mac OS X's file system isn't. As a result, if the file was named ``CHANGELOG.TXT``, and the release tool called ``git add CHANGELOG.txt``, Git would silently fail to add the file without any error codes, and the release tool would incorrectly succeed with a partial release.
- Fixed a typo in the changelog editor comments.

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- Add changelog details to commit release message
- Updated release tool to work with branches other than master
- Refactored to use ``{}`` / ``format`` instead of ``%s`` / ``%`` and fixed a bug with the new changelog feature
- Improved changelog feature to accept built-up changelog, gather commit messages, edit message in advanced editor
- Added support for plugins that can execute hooks at various stages of the release lifecycle
- Updated tool to fail more cleanly instead of erroring out on problems, check if tag already exists before releasing

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- Fixed errors that appeared if called from subdirectory.