Implement folder upload

[banksemi]

Folder uploading was only applied to NgxDragAndDropDirective.

It does not work in IE because of limitations of web browsers.

Users need to setting DroptargetOptions.folderAccept

export const ngxDropTargetOptions: DropTargetOptions = {
    color: 'dropZoneColor',
    colorDrag: 'dropZoneColorDrag',
    colorDrop: 'dropZoneColorDrop',
    multiple: true,
    folderAccept: true
};

2. If you want to get path of file, use item.filePath

var item : FileItem = ...
console.log(item.filePath);

[wKoza]

I have reviewed this PR but it's not TypeScript friendly. I keep in mind the idea and work out on it in the coming weeks