enable Usage.init_artifact to accept kwargs that are passed to the factory

[gregcaporaso]

Is there a technical reason why Usage.init_artifact doesn't accept kwargs that can be passed through to the factory function?

If not that would be convenient to enable, to support factory functions like the following:

def dna_artifact_factory(sequence_id: str, sequence: str):
    seq = skbio.DNA(sequence, metadata={id: sequence_id})
    with tempfile.NamedTemporaryFile() as f:
        # write our skbio.DNA object to file in fasta format (the default)
        seq.write(f)
        # reset to the beginning of the file
        f.seek(0)
        # instantiate our format object with the fasta
        ff = SingleRecordDNAFASTAFormat(f.name, mode='r')
        # return the sequence packaged in a "SingleDNASequence" qiime2.Artifact
        return qiime2.Artifact.import_data("SingleDNASequence", ff)

def nw_align_example_1(use):
    seq1 = use.init_artifact('seq1', dna_artifact_factory, "seq1", "ACGGTTAA")
    seq2 = use.init_artifact('seq2', dna_artifact_factory, "seq2", "ATTGTAA")
...

I planned to do this while working on usage example content for Developing with QIIME 2, but remembered that I need to define a factory function per input as it stands right now.