Limit scroll_past_end to keep n lines visible

Many files have a blank newline as their last character, so the `scroll_past_end` setting and a fast scroll result in an empty window. It would be nice if we could specify n lines to keep, something like:

    "scroll_past_end": 10

This would then scroll the window but keep the last 10 lines visible. Switching this to a number would seem to be backwards compatible since the current value `true` would function the same as setting the value to 1.