introduce option to overwrite journald parser

[slomo]

Since journald might contain different formats of logs, there should be an option to set the parser associated with the data. I can now setup parser like journald_myapplication in scalyr.

[slomo]

I am not sure if that is aligned with your product strategy, but the feature would be supper useful. Maybe this would even make sense on ScalyrMonitor inside https://github.com/scalyr/scalyr-agent-2/blob/master/scalyr_agent/scalyr_monitor.py in order to be available to all monitors (I guess shell monitor would also profit from this change).

At least for journal it is a super important feature because in my opinion journal is more a log transport than a specific kind of log, which can contain many log formats. In my particular case some 3rd party but also my own applications are logging into journal. I use structured log, but the thirdparty applications have "regular" log formats.