Add an option to have a scroll bar

[milkowski]

The scroll bar is very useful when terminal history size is quite big and allows for fast intuitive navigation.
It would be very helpful to have simple portable (not necessarily native) scroll bar on the side just like thin movable rectangle, that can be even hidden until pointed by mouse.
The simplicity of implementation just as plain thin rectangle on the side could also allow for easy color customization and ability to hide for example.