Split create certificate process into two steps #43

Hi, Bruno Albino here. I work for Maestro company, California - L.A. We start using greenlock library to create custom domains with DNS challenges. However, since we are asking to our clients manually put the records into his domain service, we can't create the certificate right after the call in the challenge.set. Our flow would be:

Client submit his domain
We validate, and then call greenlock to "add" that domain (but no create the certificate yet, just to acquire the DNS challenges.
Return to the client the challenges
Client add manually and then try to check that status
With greenlock, we need to grab that first acquired challenge and try to validate again