elfeed-search doesn't set the mark when showing an entry using RET key

[daym]

Right now, elfeed-search doesn't set the mark when showing an entry using RET key.

That means that back-button cannot go back to the line inside *elfeed-search* that was opened. In fact it doesn't even go back to the *elfeed-search* buffer at all but rather it goes back to a random buffer you certainly didn't mean.

It would be nicer to set the mark. In addition to back-button being able to go back to the original line inside elfeed-search, the user can also go back to the original line easier.