curate format-dates : --expected-date-formats defaults cannot be overridden

[victorlin]

continuing a side discussion from #1654

Problem

--expected-date-formats was originally written without defaults (empty list). Defaults were added in adf1e78, however due to use of action='extend', they cannot be overridden.

Possible solutions

1. Change behavior so that --expected-date-formats overrides the defaults by using action=ExtendOverwriteDefault. This is likely to break lots of existing usage.
2. Continue letting --expected-date-formats append to the defaults (i.e. builtins) and add another way to say you don't want the defaults (e.g. --no-builtin-date-formats flag or --override-expected-date-formats=FMT … option with args or something along those lines).

[victorlin]

In in #1709 (comment), @joverlee521 clarified that the defaults were added after adf1e78, so I've reworded the issue description. Directly relevant discussion is also happening in that thread.

[victorlin]

#1709 addresses the first part of option 2:

2. Continue letting --expected-date-formats append to the defaults (i.e. builtins) …

As for the second part,

… and add another way to say you don't want the defaults (e.g. --no-builtin-date-formats flag or --override-expected-date-formats=FMT … option with args or something along those lines).

my vote is for --no-builtin-date-formats since I think having both --expected-date-formats=FMT … and --override-expected-date-formats=FMT … is slightly more confusing and requires more implementation logic.