Add help text to clarify extend actions

[victorlin]

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'])
  

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 72.72%. Comparing base (a57d50c) to head (3d71c25).

Files with missing lines	Patch %	Lines
augur/argparse_.py	25.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1705      +/-   ##
==========================================
- Coverage   72.82%   72.72%   -0.10%     
==========================================
  Files          79       79              
  Lines        8272     8286      +14     
  Branches     1691     1697       +6     
==========================================
+ Hits         6024     6026       +2     
- Misses       1960     1972      +12     
  Partials      288      288              

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[genehack]

didn't test, code looks okay