Allow version to be propagated globally through subcommands

[kbknapp]

Another issue pointed out by SShrike, so more thanks to him! This also goes along nicely with #156 :)

If one does wish to have versions available to all subcommands, but wishes to use the same version throughout, it's tiresome to repeat App::version(&str) for all of them, especially if you have a lot or non-standard versions.

This issue requests an App::global_version(bool) which will propagate the App::version value down through the command hierarchy.