Implement support for requesting reviewers

[zackproser]

Description

Closes #15.

These changes implement support for requesting reviews from GitHub users or GitHub teams when git-xargs opens pull requests on your behalf.

Two new flags are introduced for this purpose:

• --reviewers, which accepts a comma-separated list of GitHub usernames
• --team-reviewers which accepts a comma-separated list of GitHub team names such as gruntwork-io/team-dx

Note that there are some important limitations on the ability to request reviews, as noted here in the official GitHub guide.

Namely:

The ability to add multiple pull request reviewers or requests reviews from teams is available in public repositories with GitHub Free for organizations and legacy per-repository billing plans, and in public and private repositories with GitHub Team, GitHub Enterprise Server, and GitHub Enterprise Cloud. For more information, see "GitHub's products."

That said, the --reviewers flag should generally work for GitHub free users.

TODOs

Read the Gruntwork contribution guidelines.

• [x] Update the docs.
• [x] Run the relevant tests successfully, including pre-commit checks.
• [x] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• [x] Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

Introduce support for requesting reviewers when opening pull requests.

Migration Guide

[zackproser]

Thanks for review! Going to merge this in now.