Automatic updates

[insidegui]

This is kind of related to #5 : we should add automatic updates to the app. This is kind of problematic because the app is just a container for the Xcode extension, but I think it's doable.

I always use Sparkle on my apps (no need for a package manager, it works fine with just submodules haha). It only requires a XML file with feed of updates and the binary releases, both can be stored on the repo itself.

[Bunn]

We'll need a macOS app either way because of this: #4 so it will do a bit more than just be a container for the extension.