Support getting screenshot as stream instead of file

[TwitchBronBron]

Add support for getScreenshot() returning a buffer instead of a file path, for apps that would leverage the buffer directly instead of needing to load the file from disk.

[TwitchBronBron]

This might be a non-breaking change depending on how we approach it, but it would be nice to pick the correct design during v4 which might be a breaking change.