Read stderr

[TimJentzsch]

Right now, we only read from the processes' stdout, but there is currently no way to also read stderr.
We will need another task to read that stream and also generate events for the output.

The main challenge will be to integrate this well into our existing APIs.
Maybe an application will want to read both stdout and stderr, in the same order in which they were written.
In that case, two separate event streams might not work anymore.