Add shell completion command by bumping cobra to 1.8.1

[hougesen]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Problem/Feature

This PR updates the version of github.com/spf13/cobra to the latest (1.8.1), since it has support for automatic shell completion generation.

Merging this will close #336.

Description of Changes:

• Updated github.com/spf13/cobra to v1.8.1
• Documented usage of completion command

Checklist

• [x] My code follows the Contribution Guide
• [x] I have self-reviewed the changes being requested
• [x] I have made comments on pieces of code that may be difficult to understand for other editors
• [x] I have updated the documentation (if applicable)