Optionally turn off Twitter and MessageUI dependencies

[zakdances]

If I'm not going to use the sharing features features of VideoPlayerKit, I'd like an option to add it to my project with needing to include either of those frameworks. Maybe a linker flag could be used to communicate that the developer doesn't want to use sharing dependencies?

[ArtFeel]

I totally agree. I'm using another sharing solution in my app, but VideoPlayerKit add ShareThis as dependency. As a result, I have conflicts with versions of Facebook-SDK when using cocoapods.