Add varying degrees of verbosity

[Jcpetrucci]

I'd like to see the results such as the total number of certificates fetched, but all of the "Fetching certificate ##### of #####" messages that appear when using the -verbose flag are too chatty. With regular verbosity, certspotter obviously does not display anything unless there's a watchlisted certificate found, so that no output is generated and nothing is e-mailed to the user. Can we have a level in between silence and firehose?

[paravoid]

+1. In the latest Debian package we run certspotter from a systemd service & timer. It'd be neat if a "systemctl status certspotter" showed something that wasn't total silence, so that users have some level of confidence this thing is doing something :) I found the -verbose output too chatty too, which is why I did not enable it in the service.