Option to silence non-error log messages on successful CLI run

[mhoran]

It'd be great if there were a --quiet or similar flag to silence non-error log messages when lego successfully issues or renews a certificate. I've implemented this in a wrapper script for running Lego via FreeBSD periodic, but it's a bit clunky.

The use case is this: peroidic runs via cron, and can be quite noisy. Silencing output of successful jobs is generally desirable. There's even an option to do this by default. However, to provide a uniform experience for all users, I've added the above hack to the script.

I'd be happy to take a pass at this if it's agreed that this is a valuable feature. I also realize that the CLI is undergoing a refactor, so perhaps this work should wait until that's complete.

[lingfish]

I'd love this.