Adding fields to message data

[gerritholl]

In our trollstalker processing, we add fields to the messages using configuration entries such as var_platform_name=NOAA-20 or var_orbit_number=0000. This was necessary for a reason I don't remember. For a migration from trollstalker to pytroll-watchers, we would probably need this capability in pytroll-watchers as well, or a different solution to the same problem.