Allow -o option to target one of multiple output streams
Mentioned during the art / LArSoft course: the -o option should be able to specify to which of several configured output streams should be affected by a particular file name specification.
There exists already a semantic for this: specifying particular types of stream-specific SAM metadata involves a syntax
<output-module-label>:key=value. We could use
-o <output-module-label>:<file-name> and be consistent with that.