'Opens a resource from the application\'s instance folder
(:attr:`instance_path`).  Otherwise works like
:meth:`open_resource`.  Instance resources can also be opened for
writing.
:param resource: the name of the resource.  To access resources within
subfolders use forward slashes as separator.
:param mode: resource file opening mode, default is \'rb\'.'

def open_instance_resource(self, resource, mode='rb'):

    return open(os.path.join(self.instance_path, resource), mode)
