Add version flag

[thampton]

Make the following return the version.

  ./octopus --version

Fixes #269

[CLAassistant]

All committers have signed the CLA.

[OzoneNZ]

@benPearce1 any chance of getting this reviewed and merged? We pack the octopus CLI into our custom worker-tools container image, and as part of that build process we really want to emit version info which we were able to do with the legacy CLI

[benPearce1]

@OzoneNZ there is already a version command available, just in a slightly different format

❯ octopus version
2.1.0