Add command line option --version

[fingolfin]

Resolves #3735

I guess we could try to add a test, but I am not quite sure what to test for... Perhaps a non-zero exit code, and then match the output against a regex?

[coveralls]

Coverage increased (+0.007%) to 93.735% when pulling 3c7b9c8 on fingolfin:mh/version into 7b98f48 on gap-system:master.

[wilfwilson]

How would you even go about adding a test? Would it have to go in etc/ci.sh somehow? Anyway, if you feel like doing it, it would be nice to have, but I'm satisfied without a test.

[fingolfin]

Yeah, in etc/ci.sh -- indeed, not very much to test, though one could test that the exit code is as desired, and that the output consist of a single line matching a certain pattern (as opposed to an error message or so). But I am not sure how useful that'd really be... so unless somebody reaaaallly wants it, I'd rather not invest the effort :-)