elfeed-search: Make the header function customisable
elfeed-search: Make the header function customisable

See #111.

Signed-off-by: Gergely Nagy <6780fac3b38619ac20422286e4110a70dbba2203@cloudera.com>

holomorph/elfeed,holomorph/elfeed,skeeto/elfeed,algernon/elfeed,seblemaguer/elfeed,emacsmirror/elfeed,algernon/elfeed,skeeto/elfeed,emacsmirror/elfeed,seblemaguer/elfeed