DQN Agent's Snapshot Directory

Hi, I am using your Tensorflow implementation of a DQN agent in a docker development environment. I have found that the `Snapshotter` object which creates the snapshot of the network after training is initialized with the `directory` argument set to its default value: `~/acme`: https://github.com/deepmind/acme/blob/41ea85244bc67b0ca618482cacb70a4757f2714d/acme/agents/tf/dqn/learning.py#L103

Since I am working in a Docker Container, the only way that I have found to recover the snapshot is to use a volume to bind `~/acme` to a local folder. It would be nice to be able to define the snapshot's folder just like we can define the checkpoints' folder: https://github.com/deepmind/acme/blob/41ea85244bc67b0ca618482cacb70a4757f2714d/acme/agents/tf/dqn/agent.py#L63

Thank you for this great RL library!