New feature: ability to disable "upgrade to premium" prompts outside of Play Store

[wikey]

As a longtime FBReader user who depends on FDroid for my free software applications, I would love the ability to disable the new prompts suggesting that I buy a version from the play store. I would gladly pay for some keycode to enter to turn that prompt off, even if that is the only one of the premium edition's features I can get without using Google's App store.