Provide a Flatpak as software package — for Linux systems

[Flatpak](http://docs.flatpak.org/en/latest/introduction.html) is a technology for building, distributing, installing and running applications – primarily targeted at the Linux desktop and desktop applications.

Would be great (and convenient) to be able to install `ByteSync` from `Flathub`. It would also acquire _visibility_ in the app stores automatically.