Create a data URL representing an image from a PIL.Image.

  Args:
    image: a numpy
    mode: presently only supports "data" for data URL

  Returns:
    URL representing image

def _image_url(array, fmt='png', mode="data", quality=90, domain=None):
  """Create a data URL representing an image from a PIL.Image.

  Args:
    image: a numpy
    mode: presently only supports "data" for data URL

  Returns:
    URL representing image
  """
  supported_modes = ("data")
  if mode not in supported_modes:
    message = "Unsupported mode '%s', should be one of '%s'."
    raise ValueError(message, mode, supported_modes)

  image_data = serialize_array(array, fmt=fmt, quality=quality)
  base64_byte_string = base64.b64encode(image_data).decode('ascii')
  return "data:image/" + fmt.upper() + ";base64," + base64_byte_string