Convert base64 encoding to png.

        Strips msg of the base64 image encoding and outputs
        the images to the specified directory.

def build_image(self, msg):
        """Convert base64 encoding to png.

        Strips msg of the base64 image encoding and outputs
        the images to the specified directory.
        """
        result = self.find_message(msg)

        if result is None:
            return

        self.index += 1
        images_path = 'images'

        if self.directory:
            images_path = os.path.join(self.directory, images_path)

        if not os.path.isdir(images_path):
            os.makedirs(images_path)

        with open('{0}/output_{1}.png'.format(images_path, self.index), 'wb') as fh:
            self.write_image_to_disk(msg, result, fh)

        self.out.append(
            '\n![png]({0}/output_{1}.png)\n'.format(images_path, self.index))