Introduce cobra for postcli

[fasmat]

The code in postcli is now complex enough that it becomes hard to follow which flags are intended for which commands and how everything fits together.

• This needs to be cleaned up preferably with a library like cobra.
• Existing code should be split into sub commands (init, verify, resize, bench, etc.)
• The sub commands should have some rudimentary tests
• A new sub command version should be added that prints the version of postcli/post.