Feature: offset setting for `z*`

[ttytm]

I would like to be able to further customize scrolling motions in neovim. Specifically the z motions.

E.g. that zt won't go to scroll-off but would stop a little below. Such an offset could be implemented as far as I can see fairly easy into the current codebase. If it helps - and if you can imagine implementing something like this - I could make a PR in the coming days.

I think this would give neoscroll another nice and distinct functionality. For some maybe just one, or one more reason to use it.