add standard --version argument

this way the version can be discovered by scripts without having to
part the output of 'help'.

it is removed from the 'help' output itself because it is prettier
without the complete version number, and then the description can be
reused elsewhere as well without needing the version number

borg/archiver.py

@@ -564,7 +564,9 @@ Type "Yes I am sure" if you understand this and want to continue.\n""")
         common_parser.add_argument('--remote-path', dest='remote_path', default=RemoteRepository.remote_path, metavar='PATH',
                                    help='set remote path to executable (default: "%(default)s")')
 
-        parser = argparse.ArgumentParser(prog=prog, description='Borg %s - Deduplicated Backups' % __version__)
+        parser = argparse.ArgumentParser(prog=prog, description='Borg - Deduplicated Backups')
+        parser.add_argument('-V', '--version', action='version', version='%(prog)s ' + __version__,
+                                   help='show version number and exit')
         subparsers = parser.add_subparsers(title='Available commands')
 
         serve_epilog = textwrap.dedent("""