Support animations

To visualize animations in notebooks on

```python
from IPython.display import HTML

[...]

ani = matplotlib.animation.FuncAnimation(fig, animate, frames=FRAMES)
plt.close(fig) # prevent ipython from drawing the static figure
HTML(ani.to_jshtml())
```

It would be nice to adapt the logic so that if the return value of the last statement is an Animation, automatically turn that animation into HTML and display it, so that the user doesn't have to do all the above boilerplate and can just write

```python
matplotlib.animation.FuncAnimation(fig, animate, frames=FRAMES)
```