Fix issues with message history scrolling

[cfcs]

• It shouldn't be possible to scroll beyond the last message in the history buffer.
• New messages shouldn't reset or offset the scrolling, neither should they be marked as read if they haven't actually been displayed to the user
• There should be a command for controlling the scrollback state: "/scroll reset", "/scroll [to line x]", maybe some kind of search functionality (like irssi /lastlog)
• An individual scrollback state should be preserved for each window; tabbing away to a different user, then back, shouldn't reset the scrollback state.

[cfcs]

• And perhaps there should be some overlap when scrolling, instead of scrolling entire window frames at a time

[hannesm]

happy to accept pull requests, but not really of importance for myself

[cfcs]

It was primarily a note to self :-)

[cfcs]

https://github.com/cfcs/jackline/commit/12773311c77aaf962b8609f9cade975c43f5b41c addresses above bulletpoints except the /scroll command

[cfcs]

https://github.com/cfcs/jackline/commit/743d5f655fc1f4a2f3b21eef890d6a17154ce13b implements the /scroll command