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

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

[Xemdo]

Since we can detect what system/shell you're on anyway, should we consider doing this automatically using Cobra's completion generation functions instead?

[hougesen]

Are you talking about removing the need for specifying the shell when running twitch completion, or automatically generating shell completions on install?

If the former, I would personally expect all users that wish to have shell completion to already know which shell they use, reducing the need for simplifying the command.