v.1.4.0 Watch a folder for new files

• New feature: watch a directory for changes and optimize any new image file as soon as it is created (no multiprocessing for now, sorry). This feature is not compatible with Pythonista for iOS, since it requires a third-party package called Watchdog, which is currently not available on Pythonista).
• Long running tasks, like optimizing recursively a folder containing a large number of image files, or watching for new files a a directory, can now be interrupted by the user with CTRL+C. Instead of an ugly traceback, the final report is presented with the usual statistics for the images that were processed.
• Added the ability to specify the number of tasks to run simultaneously, so that you can optionally limit the number of processor cores being used by this application and keep in reserve some CPU power for any other tasks.