Optionally clone a shallow copy of Big Bang during configuration setup

During bbctl config init, we prompt the user to input the path to their Big Bang repo checkout. Some users may not have Big Bang checked out. Big Bang is a dependency of using bbctl, so we should make sure that we appropriately handle this scenario.

Acceptance Criteria

• During configuration setup, if the Big Bang path is left blank, prompt the user to choose a directory to clone into
• shallow clone Big Bang into the configuration directory specified and appropriately configure the setting in the config.yaml