Override pdf.js to enable VIM-like scrolling by j, k, J, K, H, L

[taquangtrung]

Hi,

I override some default keys of pdf.js to enable a VIM-like scrolling experience:

• j, k for a small vertical scrolls like VIM (by default in pdf.js, j, k will scroll pages)
• J, K now scroll pages
• H, L for small horizontal scrolls (I didn't assign to h since it is used by pdf.js to select the hand tool).

Can you consider merging this PR?

Thanks!

[James-Yu]

I've modified your PR a bit:

• Instead of changing the default behavior of j and k, I'd rather suggest adding new shortcuts shift+jkhl for small navigations to avoid changing current behavior.
• The previous implementation is verbose.