Description of proposed changes

Done programmatically by replacing ArgumentDefaultsHelpFormatter with a new CustomHelpFormatter.

Related issue(s)

• Closes Improve help text of CLI options that use extend and ExtendOverwriteDefault actions #1654

Checklist

• Automated checks pass

• Check if you need to add a changelog message

• Check if you need to add tests

• Check if you need to update docs

• Tested locally with output of augur curate format-dates -h:

    --metadata-delimiters METADATA_DELIMITERS [METADATA_DELIMITERS ...]
                          Delimiters to accept when reading a plain text metadata file. Only
                          one delimiter will be inferred. Specified values will override the
                          default list. (default: (',', '\t'))
  
    --date-fields DATE_FIELDS [DATE_FIELDS ...]
                          List of date field names in the record that need to be standardized.
                          (default: None)
  
    --expected-date-formats EXPECTED_DATE_FORMATS [EXPECTED_DATE_FORMATS ...]
                          Expected date formats that are currently in the provided date fields,
                          defined by standard format codes as listed at
                          https://docs.python.org/3/library/datetime.html#strftime-and-
                          strptime-format-codes. If a date string matches multiple formats, it
                          will be parsed as the first matched format in the provided order.
                          Specified values will extend the default list. (default: ['%Y-%m-%d',
                          '%Y-%m-XX', '%Y-XX-XX', 'XXXX-XX-XX'])