Enriching the Git-Mastery root configuration with more information

[woojiahao]

Based on git-mastery/exercises#122

I noticed that an increasing number of our operations rely on the gh auth status call to retrieve the current user's username and then making calls based on that. I think this is pretty inefficient, because we have to manually execute the command and then parse the string which could be None.

My initial thought is to store this information when the user first initializes Git-Mastery, storing it under the .gitmastery.json file.

There may be other use cases, but this is the primary one I can think of right now.