Add max file size to the export settings (for example Twitter's 15 Mb)

Hey,

I use the app to create GIFs for Twitter. I often face an issue that my GIFs are just over Twitter's 15 Mb limit. I often try to crop the video a bit, but for some reason even reducing the length by 10-20% barely reduces the output size of the file.

Ideally we'd be able to either specify ideal output file size or a 'quality' slider, which would affect the expected size of the file (which btw is not really accurate for GIFs, from my experience it's +- 70%).

Great app and good luck!
Michał