[Feature request] Add custom fields

[karlvlam]

Since Logmatic store all logs into one database, it should let user to add some custom fields to filter their logs that they can work with legacy system happily.

e.g. --field=env=production --field=app=webserver
@gpolaert

[gpolaert]

You should be able to use the docker labe's. I think, it's the smartest way.
Could it suit you?