Add keybindings inspired from weechat

Hello,

I'm currently using weechat, but have been told that glirc might be a nice replacement. Having skimmed the wiki, here are the things that are blocking me from trying it, hoping you would find them useful too:

• The go.py plugin for weechat, plugged on Alt-G (or whatever), that starts fuzzy-matching the buffer list looking for a buffer to jump into
• Alt-U, that jumps to the last message not yet seen (marked by a bar in weechat, can't see it in the screenshot but I guess there's one in glirc too)
• Alt-N and Alt-P, which jump to next/previous highlight in the current buffer

In addition, something that would be nice to have (but which weechat doesn't have) and that I can't find in the docs either is a way to reset the “last read message” bar to some other position, if one wants to mark a message as unread to come back to it later.

What do you think about these? :)

No files, tests, or entry points are named. Begin by locating the client’s keybinding and unread-message behavior, then clarify the scope and acceptance criteria with maintainers; done would cover the three requested navigation bindings and repositioning the last-read marker.