Add GitHub enumeration capabilities

[bradlarsen]

This adds rudimentary GitHub repository enumeration capabilities to Nosey Parker.

• A new github repos list command is available, which uses the GitHub REST API to enumerate repositories belonging to one or more users or organizations.
• The output can be written in textual, JSON, and JSON lines format.
• An optional GitHub personal access token can be supplied, which provides a much greater rate limit than with unauthenticated access, and may make additional repositories accessible.

Fixes #15.

[github-advanced-security]

clippy found more than 10 potential problems in the proposed changes. Check the Files changed tab for more details.