add mechanism to return current version number

[LucasLarson]

gunstage --version

should return something like

https://github.com/LucasLarson/gunstage/blob/4aae64ac16e42c30efbd8e6a65eb5077c437bebf/citation.cff#L13

or alternatively, something like what git describe returns on a clone of this repository

v1.8.2-22-gae1db6d973