Progress Callback

When using the progress_callback parameter of the render_list() method, it’s possible to inform others about the progress of string generation. This is especially useful when generating a large number of strings.

The callback function obtains two int parameters: (current, total), which define the current progress and the total amount of requested strings.

By using that, callers of render_list() are able to implement a progress indicator suitable for informing end users about the progress of string generation.