Accelerate scroll on multiple key presses

[s1n7ax]

When user repeats the same key before the scroll is done, https://github.com/psliwka/vim-smoothie scrolls the same amount of lines but within the time of a single scroll instead of two scroll time. Similar feature would be nice in neoscroll as well.

Scenario 1:

• User hit <c-d>
• Neoscroll scrolls 10 lines in 500 ms
• User hit <c-d> once again after the scroll is completed
• Total lines scrolled is 20 lines and it took 1000 ms

Scenario 2:

• User hit <c-d>
• User hit <c-d> once again when only 5 lines has scrolled within 250 ms (before scroll is completed)
• Neoscroll scrolls 15 lines 500ms
• Total lines scrolled is 20 lines and it took 750 ms

[karb94]

This might conflict with the "stop on key release" feature. However, I can see some value in this. I think it makes more sense to speed the animation on the 3rd <C-d> rather than on the 2nd as it could mess up scrolling animations with easing functions (after the 2nd <C-d> the scrolling is constant at max speed).

This is a non-trivial change so I'll need to find some time for it.