doesn't support icon_url

[masukomi]

in addition to icon_emoji Slack supports icon_url

I'd like to see icon_url be supported via the conf file. Best would be if it would just take whatever was in that file, override anything set via the command line options, and then send that to the server. That way you've got support for new any new functionality they add WITHOUT having to update the code (until you want it to be overrideable on the command line.)