Support --preferred-chain like certbot

[lukecyca]

From nginx-proxy/acme-companion#695:

Let's Encrypt is changing the default chain of trust (details), which has wide implications for backwards compatibility for clients with old CA root stores. Certbot now supports specifying --preferred-chain so that we can manage this transition (details).

The first part of this transition is going to occur in less than a week, or as soon as they receive their cross-signature from IdentTrust for the new "R3" Intermediate. So this is very time sensitive.

Will simp_le be supporting this option?