How to render performance of agent visually

It would be nice to show how the agent is performing inside the game, but in the meantime here's some imperfect but functional code.

Note: This dramatically slows the throughput of the model. Only render after the model has finished training. May require additional dependencies (like matplotlib) if not already present.

Inside dopamine/atari/run_experiment.py:
```
def _run_one_step(self, action):
"""Executes a single step in the environment.

Args:
action: int, the action to perform in the environment.

Returns:
The observation, reward, and is_terminal values returned from the
environment.
"""
self._environment.render(mode='human') #Shows game in progress. Note: Slows down training tremendously.
time.sleep(1/60)
observation, reward, is_terminal, _ = self._environment.step(action)
return observation, reward, is_terminal
```