support for horizontal scroll

[hatelove85911]

Is it possible to support horizontal scroll?

[yuttie]

@hatelove85911 Yes, it is possible.

As a proof of concept, you can try this config:

let g:comfortable_motion_scroll_down_key = "zl"
let g:comfortable_motion_scroll_up_key = "zh"

We need to add another state variable for animation on the horizontal axis. It is also needed to make functions tick() and flick() more generic so that they can be used for the simulation on the both axes.