Implement structured logs using logrus

This was based loosely on an earlier implementation by
Danny Kopping <danny.kopping@grafana.com>

smtprelay.ini

@@ -3,6 +3,12 @@
 ; Logfile
 ;logfile = /var/log/smtprelay.log
 
+; Log format: default, plain (no timestamp), json
+;log_format = "default"
+
+; Log level: panic, fatal, error, warn, info, debug, trace
+;log_level = "info"
+
 ; Hostname for this SMTP server
 ;hostname = "localhost.localdomain"