Output widget

Ping @ellisonbg because I said I would and @jasongrout who originally requested this.
I'm using it like a stack.  This allows the user to do this:

```python
a = Output()
b = Output()
display(a, b)
print("normal (1)")
with a:
    print("to a (1)")
    with b:
        print("to b")
    print("to a (2)")
print("normal (2)")
```

I'll add a comment saying so.