Add version option

[S30tt]

Is your feature request related to a problem? Please describe.
No, this is a new feature.

Describe the solution you'd like
When running packj -v or packj --version it will display the version number of the tool.

Describe alternatives you've considered
Passing a random string as the command to the tool returns and error. The prefix of the error is the app version string. I am considering parsing this but a dedicated command -v --version would be more standard.

[ashishbijlani]

Thanks! Commit 6e7eca8 takes care of this.