Multi-git is a Cobra application

Refactored multi-git to use Cobra commands

Testing 1,2,3

end to end tests

Command-line interface refactoring

Move main.go into cmd/mg

Core logic refactoring

Moved core logic to repo_manager package
Added a helpers package
Added unit tests for repo_manager

Simple multi-git command-line program

This is a very basic command-line program that lets you run any command against a set of git repositories, but doesn't have any bells and whistles.