request + bug(s): add prop to specify default shimmer height

[dougg0k]

the title

I noticed that the shimmer seems to change sizes, when the loading is finalizing, shimmer item change to a smaller height, would help to keep the same height size always.

I tried specifying a custom placeholder component, but only showed a single shimmer item, instead of in each of the spaces waiting to be loaded.

Also if the list is long, it's like the shimmer doesnt fill all of the spaces if you continue to scroll, I got a half shimmer once, and when I continued to scroll without stop, it was blank space, that's probably another bug.

Edit: I see why, if I quick scroll and there is only a handful of items, it's like your shimmer tries to squeeze to fit, therefore it get smaller, it's like a fixed image, can you make so it's component based on the amount of items waiting to be loaded given where you are on the scroll?

I'm using v1.4.2