Add measure for repository default branch as not master

[d33bs]

Description

This PR adds a measure to check whether the default branch is not named master (can be named anything besides this).

Thanks for any feedback!

Closes #119

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 ! I've made a few changes based on your feedback - namely that we now look for a remote head and matching master reference. Merging this in now.