Provide a "pre-process" hook for to-master

[jdigger]

It would be very useful to provide a way to hook into the to-master lifecycle, before attempting to push to a central repository.

Examples, depending on your workflow:

• Verify commit messages have an issue id
• Verify there's no -SNAPSHOT versions
• Verify the last commit has a "Signed-off-by:" message

It would make sense that it can look in certain default locations, such as .git-pre-to-master in the root of the repository for project-specific validations, and .git/hooks/pre-to-master for "traditional" hook functionality.