[ENH] Support Dublin Core for dates

[jtojnar]

Is your feature request related to a problem? Please describe.

The RFC822 date format used by RSS is too complicated and providers often make mistakes, making the date unreadable (noticed in fossar/selfoss#1384 in https://www.commitstrip.com/fr/feed/).

Describe the solution you'd like

If there is an alternative in a less obnoxious format, we should prefer it. One such format is Dublin Core, which is pretty rare so I would say that if someone takes the time to implement it, they will likely implement it properly.

We would care about the following two elements:

• {http://purl.org/dc/elements/1.1/}created
• {http://purl.org/dc/elements/1.1/}modified

We already support the {http://purl.org/dc/elements/1.1/}date element, which is the less specific.

Describe alternatives you've considered

We could do nothing.

Additional context

The format of the value is only a recommendation and can contain a date, date/time, or period of time according to ISO 8601-1 or a published profile of the ISO standard such as RFC 3339 (which the example feed uses).

I can submit a PR.