TriggeringView doesn't update threshold if renderHeader() component updates its height

Hey, first of all, thank you for this amazing work!

I'm using it in a special scenario where the header component height is calculated dynamically (using [react-native-auto-height-image](https://github.com/vivaxy/react-native-auto-height-image)).

This would be my code:

import React, {Component} from "react";
import {Text, View} from "react-native";

import AutoHeightImage from 'react-native-auto-height-image';
import HeaderImageScrollView, {TriggeringView} from 'react-native-image-header-scroll-view';

const DEFAULT_HEADER_HEIGHT = 500;

export default class WallScreen extends Component {

componentWillMount() {
this._setHeaderHeight(DEFAULT_HEADER_HEIGHT);
}

render() {

return (








Lorem ipsum dolor sit amet, consectetur adipisicing elit. Aliquam asperiores at commodi cumque
dolor dolorum, ea eaque est excepturi nam, perferendis perspiciatis, porro! A illo ipsa itaque
quae rerum unde?





);

}

_setHeaderHeight(height){
// Here I would dispatch the redux action to update 'headerHeight'.
}

_renderHeader() {
return (



);
}

_onHide() {
console.log('On hide called');
}

_onDisplay() {
console.log('On display called')
}

}

Until now, all great! All the layout works great out of the box and my code updates the header height correctly!

BUT the problem comes with `TriggeringView`.
As you can see, the header height is initialized at `500`. Well, as I update the `headerHeight` prop via redux, `TriggeringView` will still trigger the `onHide` and `onDisplay` callbacks using the `500` initial height.

I took a look at the source and saw that the triggering threshold is based in the `initialPageY` property, and this property is calculated in the `onLayout` hook.
So the problem is this that hook is not triggered when the header height changes.

PS: I fixed this adding a new `dynamicHeaderHeight` prop to `HeaderScrollView`. Will submit a PR today!

Thanks!