Add an option to disable modeline from elfeed-search buffer in case of horizontal split

[nixmaniack]

When I have horizontal split(the literal meaning, not emacs one) with *elfeed-search* and *elfeed-entry*, the modeline in the *elfeed-search* buffer isn't much useful as there's nice header.

[algernon]

I have a reasonable idea how to implement this, but that depends on #7.