defaults not listed in cli docs

[nick-youngblut]

For much of the cli help docs, the defaults are not included. For example:

Heuristic Detector Options:
  --min-frequency MIN_FREQUENCY
                        Minimum frequency required to retain a k-mer.
  --min-contaminant-match-frac MIN_CONTAMINANT_MATCH_FRAC
                        Minimum fraction of nucleotides that must align for a
                        detected contaminant to match a known adapter
                        sequence.

Why now use an argparse formatter that automatically lists the defaults? For example:

class CustomFormatter(argparse.ArgumentDefaultsHelpFormatter,
                      argparse.RawDescriptionHelpFormatter):
    pass

parser = argparse.ArgumentParser(description=desc, formatter_class=CustomFormatter)