Render read_marker

[Minnozz]

WeeChat has a weechat.look.read_marker setting that renders a line above the first unread line in a buffer, making it easy to find the place to start reading new messages. It would be nice if Lith also rendered this marker.

[Minnozz]

For reference, Glowing Bear uses the hotlist info to determine what the last seen message is:
https://github.com/glowing-bear/glowing-bear/blob/6cd59b76d71d3fd1c765b3b9e2bd9f7bc0e409bd/src/js/handlers.js#L413-L437