feat: add shell completions

[Ulexus]

Adds completion command for all cobra-supported shells.

Fixes #21

Signed-off-by: Seán C McCord ulexus@gmail.com

What? (description)

Adds autogenerated shell completions via the completion subcommand.

Why? (reasoning)

Issue #21

Screenshots (if applicable)

GitHub Issue (if applicable)

[closes #21]

Acceptance

Check your PR for the following:

• you included tests
• you linted your code
• your PR has only one commit (interactive rebase!)
• your commit message follows Conventional Commit format
• you are not reducing the total test coverage

[Ulexus]

@particledecay Do you have idea what is going on with this codeclimate issue? I haven't even touched add.go.

[particledecay]

@Ulexus sorry, been busy lately. This is why it's not passing:

Error: you must supply a CC_TEST_REPORTER_ID ENV variable or pass it via the -r flag

That secret is there, in the upstream repo... I'm guessing since that won't trickle down to your fork, it doesn't work. Looks like I might not be able to rely on diff-coverage and test-coverage to succeed from forks 😕

[particledecay]

This is great. Thanks @Ulexus so much!