Possibility to modify the format string for elfeed-summary--render-feed

[Thaodan]

It would be handy to be able to customize the format-string for elfeed-summary--render-feed similar to how Gnus allows it.
The usage for such a feature would be to customize the layout of the string besides the padding or to customize the character used to frame the unread count.

Personally I would do something like this:

       (setq gnus-group-line-format (concat "%M%S%p%P%5y "
                                             (propertize ;(all-the-icons-faicon "envelope-o")
                                              (all-the-icons-material "mail")
                                              'display '(raise 0.0))

[SqrtMinusOne]

Nice idea. I see gnus has this logic more or less abstracted, maybe I'll try to reuse that.

Added this to my enhancements list for whenever I can take it on.