Add community health file based metrics

[d33bs]

Description

This PR adds metrics for checking whether various community health files exist within a given repository. Specifically we focus on four common files: README, CONTRIBUTING, CODE_OF_CONDUCT, and LICENSE. We check for these files using a single function which I feel may be useful for other metrics in the future (for example, as in #107).

Thank you for any feedback you may have!

Closes #114
References #106

What is the nature of your change?

• Content additions or updates (adds or updates content)
• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (these changes would cause existing functionality to not work as expected).

Checklist

Please ensure that all boxes are checked before indicating that this pull request is ready for review.

• I have read the CONTRIBUTING.md guidelines.
• My code follows the style guidelines of this project.
• I have performed a self-review of my own contributions.
• I have commented my content, particularly in hard-to-understand areas.
• I have made corresponding changes to related documentation (outside of book content).
• My changes generate no new warnings.
• New and existing tests pass locally with my changes.
• I have added tests that prove my additions are effective or that my feature works.
• I have deleted all non-relevant text in this pull request template.

[d33bs]

Thanks @gwaybio ! Merging this in after addressing your comment.