[Request] ooption to disable titlebar

[pepa65]

I have applied the patch https://savannah.gnu.org/bugs/?58152 to
40053593a870601c9c346f93ab180be1d564995c and started nano with the `-0`
flag.

Loved the extra space! But I would really want a toggle command that I
can put in .nanorc with a keybinding of my choosing. (Like Brand said,
maybe no default key binding?)

To just have it off all the time, especially if I use multiple buffers,
would not be ideal, but if it would be toggleable while in session, I
would always want to start without the top line.

Cheers,
Peter