add swipeDownThreshold prop

Author: Philip K

src/image-viewer.component.tsx

@@ -472,6 +472,7 @@ export default class ImageViewer extends React.Component<Props, State> {
           onClick={this.handleClick}
           onDoubleClick={this.handleDoubleClick}
           enableSwipeDown={this.props.enableSwipeDown}
+          swipeDownThreshold={this.props.swipeDownThreshold}
           onSwipeDown={this.handleSwipeDown}
           pinchToZoom={this.props.enableImageZoom}
           enableDoubleClickZoom={this.props.enableImageZoom}
@@ -541,6 +542,7 @@ export default class ImageViewer extends React.Component<Props, State> {
               imageWidth={width}
               imageHeight={height}
               enableSwipeDown={this.props.enableSwipeDown}
+              swipeDownThreshold={this.props.swipeDownThreshold}
               onSwipeDown={this.handleSwipeDown}
               pinchToZoom={this.props.enableImageZoom}
               enableDoubleClickZoom={this.props.enableImageZoom}

src/image-viewer.type.ts

@@ -66,6 +66,11 @@ export class Props {
    */
   public enableSwipeDown?: boolean = false;
 
+  /**
+   * threshold for firing swipe down function
+   */
+  public swipeDownThreshold?: number;
+
   public doubleClickInterval?: number;
 
   /**