Replace current unpublish command

[jessehouwing]

See: microsoft/vsmarketplace#1708

The current unpublish command will perform a delete, not actually an unpublish.

With the new inability to restore a deleted extension, this si causing issues.

Recommendation:

• Replace the underlying implementation of unpublish to properly unpublish
• Add remove or delete command with a --force option with a confirmation when called interactively
• Add documentation to help to explain the severe consequences.