[Feature Request] Improve output of `--appimage-version` by adding date and commit id

When running an AppImage with *`"--appimage-version"`*, the only info which currently prints *could* be:

Version: continuous

(I dunno what the info looks if the AppImage was built with a *released* version, maybe it's a bit more specific then?) Compare this to running *`"appimagetool --version"`*. This prints:

Version: continuous-1-g63485ed

In case a user or one of the developers wants to debug something (or help a user with a problem), the second version is slightly more useful. So the same info should added to the output of *`"--appimage-version"`*.

However, even better would be to also add the *date* of the respective commit, for example like the following:

./HelloWorld-x86_64.AppImage --appimage-version

  Version: continuous-abcd9876

  Date: 2017-11-20  11:13:56 UTC

This way, a user would only have to memorize *"To have this bug fixed or have that feature added, I need a version later than Nov 20th, 2017"*.