RFC: Support for automatically generated tag messages

Instead of being prompted to provide a tag message after a hotfix / release finish, why not letting gitflow generate one? With these two new configuration options, one can choose to provide a static message with gitflow.config.tagmessage (for example "Tagging new version"), or letting gitflow create a message based on the current version number by setting gitflow.config.autotagmessage to true. The latter will take the version prefix and the version number to produce the tag message.