[FEATURE] Use ~/.mrconfig as the default list of repos

[cro]

Problem

myrepos (https://myrepos.branchable.com) is one of the more popular multiple-repo management tools. It would be nice if git-scope could optionally read the .mrconfig file instead of trying to scan for repos. Then you could have a single source of truth for a particular project.

Proposed Solution

Allow git-scope to use .mrconfig for its list of repos.

Alternatives Considered

The current implementation is still useful--it does work for me to just pick the directories to scan inside. Our tooling just keeps .mrconfig up to date for us so we don't accidentally leave repos out. In our project we are still adding repos about once a week.