dub package info added.

simple dub package.json added. It provides only two configuration:
1. "Derelict3-gl3n-SDLImage": Derelict3, gl3n and SDLImage
2. "Derelict3-SDLImage": Derelict3 and SDLImage
Other configuration can be added easily, but I didn't to avoid probable garbage because I'm not sure it is the best way to enumerate all possible combinations.