Add ability to set at least px to infiniteScrollDistance

[Sany18]

Hi, I use ngx-infinite-scroll v17.0.0

When the scroll container gets too high (for example 10+ pages), the infiniteScrollDistance percentage value starts triggering the scroll event too early, before the scroll reaches an imaginary lower bound.

Another case. For example, we have a default value of 20% to the lower limit. If we have uploaded more than 5 pages, each subsequent page will be less than 20% and the scroll event will be triggered on each scroll tick down instead of allowing the user scroll to the bottom bound.

I suggest adding CSS measurement support like rem, or px. Or at least px.