`--participant-label` is not merged with configured subject filters

As per #216, the list of subjects provided by `--participant-label` is supposed to be merged with the filters provided in the config file. Right now, if config filters are provided, `--participant-label` is just ignored.

It's possible this behaviour was intended after further discussion, I can't remember. Specifying `subject` filters is a bit unusual, I could see how if one is doing that, one wouldn't want `--participant-label` to interfere. @akhanf, do you have any memory of this?

Start by tracing how `--participant-label` is handled alongside configured subject filters, and review the behavior described in #216. Confirm the intended interaction before changing anything; done means the command-line labels are merged with config filters, or the issue is resolved with the agreed alternative behavior.

Written by the indexing model from the issue text.