Add --version and --help support to targetcli

I don't envision targetcli taking a lot of cmdline options, so just
implement by looking at sys.argv[1] instead of using getopt or ArgParse.

Signed-off-by: Andy Grover <agrover@redhat.com>
Merge pull request #43 from agrover/add-usage

Add --version and --help support to targetcli