Adds support for using Atlas/v3 API secrets when connecting to the push api.
Use --secret=xxx with Atlas/v3 secrets, for v2 the same flags as before are used: --client-id combined with --client-secret
In addition to this the code has been overhauled to make improve readability and error handling.
With this release we are now using go modules for dependency management, so golang 1.13 or later should be used.