jump-to-next-unread options

Adds the three options `openbrowser-and-mark-jumps-to-next-unread`, `toogleitemread-jumps-to-next-unread` and `markfeedread-jumps-to-next-unread` (default is false to not break the current behavior) so that `O`, `N` and `A` jumps to the next unread item instead of just the next item.