Added onEndReached and onEndReachedThreshold props

[davebcn87]

Description

Added onEndReached and onEndReachedThreshold props to be able to add pagination to our lists. You can see a pagination example in the List.tsx file. Solves #31.

onEndReached

Called once when the scroll position gets within onEndReachedThreshold of the rendered content.

onEndReachedThreshold

How far from the end (in units of visible length of the list) the bottom edge of the list must be from the end of the content to trigger the onEndReached callback. Thus a value of 0.5 will trigger onEndReached when the end of the content is within half the visible length of the list.

Usage

<RecyclerFlatList
  onEndReached={() => {
    var elems = this.state.elems;
    elems.push(...this._generateArray(elems.length, 20))
    this.setState({ elems: elems })
  }}
  onEndReachedThreshold={0.5}
/>