Date for Unreleased #634

In a project I'm using this for I've started adding the date to 'Unreleased' in the same format as for releases. I update this date whenever I update the section.

The main purpose for this is to give a consistent view of when the changelog unreleased section was updated. Ideally this will match with the commit date of the latest notable change.

For me, a secondary purpose is that many people use my project by simply cloning the repo and periodically doing a git pull to get the latest. I then extract this date in the version command so when people report bugs, I can know when their copy was last refreshed.