feature request: allow configuration of neural network via gin

[M00NSH0T]

Just a feature request. I think this would be much more easily adaptable to non-atari environments if we could configure the neural network via the agent's gin file, or something similar. I think it would also be beneficial to use keras instead of slim, if possible, for those of us who want to easily build more complex networks.

[psc-g]

thanks for the feedback! configuring the network via gin has its drawbacks, as it makes the setup a little more opaque, but we'll keep it in mind.