Add `--version` option

[pederhan]

$ mreg-cli --version
mreg-cli version 1.0.0

$ mreg-cli --help   
usage: mreg-cli [-h] [--version] [--url URL] [-u USER] [-d DOMAIN] [-p PROMPT] [-v] [-l LOGFILE] [--show-token] [--record RECFILE]
                [--record-without-timestamps] [--source SOURCE] [--token-only]
                [command ...]

The MREG cli

options:
  -h, --help            show this help message and exit
  --version             Show version and exit.

connection settings:
  --url URL             use mreg server at URL (default: http://127.0.0.1:8000)
  -u USER, --user USER  authenticate as USER (default: test)

mreg settings:
  -d DOMAIN, --domain DOMAIN
                        default DOMAIN (default: example.org)
  -p PROMPT, --prompt PROMPT
                        default PROMPT), defaults to the server name if not set.

output settings:
  -v, --verbosity       show debug messages on stderr
  -l LOGFILE, --logfile LOGFILE
                        write log to LOGFILE
  --show-token          show API token after login
  --record RECFILE      Record all server/client traffic to RECFILE
  --record-without-timestamps
                        Do not apply timestamps to the recording file
  --source SOURCE       Read commands from SOURCE
  --token-only          Only attempt token login, this will avoid interactive prompts.
  command               Oneshot command to issue to the cli.