Function to interactively modify separator's date format

[bormoge]

Originally I envisioned a function that is conceptually somewhat similar to elfeed-search-set-filter, in that both manipulate a variable that already has a default value but can also be modified on-the-fly to adjust the information displayed in the elfeed-search buffer.

The difference between the two functions would be that elfeed-search-set-filter limits what information is displayed, and this function, which could be provisionally referred to as elfeed-search-set-separator-date-format, modifies how the information is displayed.

My argument as for why include this in elfeed is that it would synergize with elfeed-search-next-separator and elfeed-search-previous-separator while providing a nice QoL; instead of using

M-: (setq elfeed-search-date-format value)

then refreshing the buffer, the user could simply press a key to make an interface appear.