Add structured logging via logrus

[JonathonReinhart]

This adds structured logging using logrus.

This PR is a result of discussions on #13 and is based loosely on grafana/smtprelay#10.

This adds two new config options to the INI file:

• log_format which can be one of:
  • default -- The default logrus format, but with full timestamps, similar to the previous output:

    INFO[2021-03-13T00:54:21-05:00] listening on address                          address=":2525"
    

  • plain -- The default logrus format, but with no timestamps (useful for running under Systemd)

    INFO listening on address                          address=":2525"
    

  • json -- JSON formatting with RFC3339Nano timestamp format:

    {"address":":2525","level":"info","msg":"listening on address","time":"2021-03-13T00:56:40.251607698-05:00"}
    

• log_level -- The logging level; defaults to "info" (can be panic, fatal, error, warn, info, debug, trace)

[JonathonReinhart]

@dannykopping Please take a look and let me know if this doesn't work for you guys. Aside from obvious log location or message changes due to the various refactorings, this differs from what you have in the following ways:

• I set the default log file to stderr rather than stdout, because I think that's more common, and also the logrus default.
• I set the default log format to "default" (plain text), to more closely match the previous behavior.
• I set the default log level to "info" based on my and @decke's preference.
• Errors are recorded to the error key, not err as before. If that's a big problem, we can set `logrus.ErrorKey = "err", but I thought it made sense to leave it at the default.

I think you should be able to run with the nearly equivalent functionality as before with this INI snipppet:

logfile = "/dev/stdout"
log_format = "json"
log_level = "debug"

[decke]

Good catch! In which kind of error situation does SendMail return this?

[JonathonReinhart]

I can't take credit for this; it was taken straight from #13. But in my testing I have seen:

ERRO[2021-03-13T02:47:27-05:00] delivery failed                               err_code=530 err_msg="5.7.0 Authentication Required. Learn more at\n5.7.0  https://support.google.com/mail/?p=WantAuthError h75sm6304163qke.80 - gsmtp" from=jreinhart@jrr-vaio host="smtp.gmail.com:587" peer=127.0.0.1 to="[jonathon.reinhart@gmail.com]" uuid=ada212ce-ca9d-4a2b-923a-830ce6f19e37

[decke]

@dannykopping do you know in which situation SendMail returns this error? I can check myself later but need a bigger screen.

[dannykopping]

Looks pretty good @JonathonReinhart - thanks for taking the time to do this!
Left a couple thoughts but overall happy with it, and kind of you to add this for us to maintain our current behaviour:

I think you should be able to run with the nearly equivalent functionality as before with this INI snipppet:

[JonathonReinhart]

Okay this should be ready to go now if @decke is happy with my updates.

[decke]

Thanks a lot guys! It was a pleasure to see how opensource can benefit us all.