add the version command

[rgl]

closes #10

this was tested by creating the v0.3.3 git version tag and then executing the following commands:

$ goreleaser release --skip=publish --clean
...
 • getting and validating git state
    • git state                                      commit=0fbcb9d92034f39ac25158bab00cedef650fc498 branch=rgl-version-command current_tag=v0.3.3 previous_tag=v0.3.2 dirty=false
...

$ ./dist/kubectl-linstor_linux_amd64_v1/kubectl-linstor version
0.3.3