Allow users to add other options for creation date extraction

[jgillula]

Unfortunately paperless-ng doesn't correctly extract that creation date for a lot of my documents. That's because my documents have dates with formats like:

1. Feb. 20, 2021 - Mar. 23, 2021
2. 02/01/21 thru 02/28/21

paperless-ng completely ignores the first type of date format because it doesn't precisely match the fixed set of regexes it uses. On the second it extracts the date as January 2, 2021--this is probably due to some locale mixup reading the mm/dd/yy date as dd/mm/yy. Even if I could fix that, it still doesn't help me because what I want is the last day of the billing cycle (February 28, 2021), not the first.

It would be awesome if we could let the user add additional options for extracting dates. I can see a couple possible ways to do it (in order of increasing complexity):

1. Allow users to add custom regexes to DATE_REGEX, via some option in the backend's config file
2. Allow users to add custom date formats using some formatting language (maybe Python's date format codes?), and then automatically build the regexes and add them
3. Similar to Different filename formats per document type #982, allow either (1) or (2), but on a per-document-type basis, falling back to the default if the document-type is none or a per-document date format hasn't been set.

As with #982 I'm willing to try to implement this and make a pull request. But I'd love to work out the design details via discussion first, to make sure the maintainers are interested in what I'd code up. :-)