Add method to clear the cache manually

[Tienisto]

Hi,

I cannot find a method to clear the cache manually. This is essentially annoying on iOS as we don't have a "clear cache" button in the settings.

The file picker package provides a method so share handler should provide a method too.

[JoshJuncker]

Sorry. I don't have the time to look at this issue right now. But please feel free to contribute and make a pull request. I can try and help answer questions too if you run into some along the way.