Gist update functionality should execute the code cell as well

It seems like it would be convenient for a code cell to be re-executed when changes to it are being used to update a gist.  For example,

``` Python
%%gist <gist_id>
print("new snippet code")
```

should actually output "new snippet code" rather than just updating the gist.  Otherwise iterating on a snippet would require removing the `%%gist` line and then re-adding it back each time.
