Feature request: pull and push with %%px cell magic

I propose allowing one to pull and push in a px cell. The feature allows better encapsulation of cells.

example:
```
%%px push(a, b) pull(c)
c = a + b
```
translates to something like
```
client.push(['a', 'b'], block=False)
%px c = a + b
client.pull('c', block=False)
```