Add scroll-step-mouse and scroll-hstep-mouse option

[dirichy]

I want to use both mouse and keyboard to scroll.
When I set scroll-step big, scrolling with my mouse is too fast, while when I set scroll-step small, scrolling with my keyboard is too slow.

I tried to use keymap to solve this, such as

set scroll-step 1
map j feedkeys "100<Down>"

This can work with both keyboard and mouse, but when I type "100j", it will be so slow.
I guess the count of a command is not just re-feedkeys in the basic layer, so aftering mapping it's just re-feeding and very slow.

I have two possible method to solve this, the first one is in the title.
The second one is that we can add a count argument in scroll command, so I can use

map j scroll down 100

or something like this.

I love this pdf-viewer very much and I hope it can be better, thank you for your work.