[Feature Request] Vertical alignment of interpreter input in terminal

Hi,

I wonder if there's an option in IPython for centering the REPL input (`>>> `) in the middle, so that it's easier to show things during screen casting. This is especially useful when terminal is set to fullscreen, because the input would stay at the middle instead of the bottom.

Example of such functionality can be seen in vim when one sets the `scrolloff` to high value. I've recorded short video which hopefully shows that clearly: https://asciinema.org/a/384896

This functionality could be enabled as a magic command `%scrolloff middle` or something similar. I am also willing to implement that if no such thing exists already, but I would need some hints.

Thanks!