add on file changed directive

[robmcguinness]

const fileChange = {
  restrict: 'A',
  link(scope, element, attrs) {
    // clean up bind on scope destroy  
    element.bind('change', () => {
      scope.$applyAsync(() => {
        const onFileChangeHandler = scope.$eval(attrs.onFileChange);
        const el = element[0];

        if (el.files) {
          onFileChangeHandler(el);
        }
      });
    });
  },
};

export default function() {
  return fileChange;
}