[feature request] Add a custom window title format

[raphaelfournier]

I would like to be able to customise the window title of termdown. It's a great idea to have the time in the title. However, sometimes I would need a different time format (e.g., not the "human readable" one), or prefix/suffix it with something else (e.g., "-- termdown"), so that my WM may match the window with termdown.

I currently changed line 591 in file /usr/lib/python3.11/site-packages/termdown.py as follows:

os.write(stdout.fileno(), "\033]2;{0}\007".format(stopwatch_text+" -- termdown").encode())

It's very custom, so I don't make it a PR as is.

Thanks for the soft!