Option for ISO formats

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature

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

dd/mm/yyyy and mm/dd/yyyy date formats can be mistaken for the other, meaning they are both equally unworkable. I usually end up going for Canadian formats to get nice yyyy-mm-dd dates, but I have a feeling it could cause problems in the future.

**Describe the solution you'd like**

A "programmer standard" formats option, meaning
* metric measurements
* sensible date and time formats
* ISO 8601 for formal dates
`2020-07-23 00:00:00`
* RFC2822 for informal dates or dates that may require day-of-week
`Thu, 23 Jul 2020 00:00:00`

**Existing work**

Not in format-pickers that I've been able to find, but the `en_DK` locale was supposedly created for this purpose.

**Describe alternatives you've considered**

Currently I use Canadian formats, when it comes to locales people commonly go for `en_IE` (English, Ireland).

**Additional context**

https://unix.stackexchange.com/questions/62316/why-is-there-no-euro-english-locale

## Notes

This may be out-of-scope and to be closed as Wontfix.