Implement scroll_lookahead and scroll_size params

[infokiller]

See below for context, and also the following which should be solved:

• [Bug Report] Up/Down shortcuts are broken when Google continuously loads new results #464 (comment)

• Trigger infinite scroll on youtube home page #389 (comment)

          I'm not sure I agree this is better (if I understand what you wrote correctly). Specifically, moving the viewport so that a result is at the top of the page will cause a big nonlinear scroll.
  

Why do you prefer the behavior you suggested?
is it because you want to see the next results without having to navigate to them?
is it because the viewport moving is disturbing you?

Anyway, I think this can possibly be solved by adding two options:

1. scroll_lookahead- an integer specifying the number of results to "look ahead" when navigating. For example, if this option is set to 0 you will get the behavior in 0.3.2 where you will see the selected result at the bottom of the page (after navigating down enough). If this option is set to 1, the scrolling will begin when the selected result is the one before the bottom.
2. scroll_size- an integer specifying the number of results to scroll when needing to scroll because of a navigation to a result outside the viewport.

Originally posted by @infokiller in #35 (comment)