Create `bbctl update bbctl` command for homebrew installations

Create a bbctl update bbctl command. This root command will be used in the future for updating the individual application charts but we should treat the bbctl target as a unique case for upgrading the client itself.

Acceptance Criteria

• When run, bbctl should print out instructions to the user to update to the latest version of bbctl using homebrew
• Support an optional --version parameter that gives the user a valid command for homebrew to install a previous version