Enable `completion` command

**Describe the problem/challenge you have**
When #126 came by, we responded by disabling the automatic inclusion of the `completion` command (see https://github.com/vmware-tanzu/carvel-kbld/pull/126/commits/5bed2f031710e1df04c6cf6d9306ce322e911fcc) because out of the box, that functionality was not compatible with `kbld`'s UI display.

**Describe the solution you'd like**
Figure out where those incompatibilities are and address them so that users can enjoy the bliss of tab-completion.

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Start by reviewing the historical change in PR #126, especially commit 5bed2f031710e1df04c6cf6d9306ce322e911fcc, to understand where automatic completion was disabled. Trace the completion command and kbld's UI display together; done means tab completion is enabled without breaking the existing UI.

Written by the indexing model from the issue text.