github release: add full support for flags and messages

Currently, the git-flows 'Finish release' does a good job and has some nice flags already. Unfortunately, it does not work well together with the github release which offers some other flags (pre-release...) and the possibility to add a description (and more...).

Without this support, a release from smartgit is quite cumbersome as it requires to create a manual release on github and delete the tagged release created by smartgit