magic idea: result

a new magic: `%result x` which takes the value returned from the preceding lines, and assigns it to the variable `x`.

then maps and plots could have a feature that took their interactive state, and turned it into code,
that was inserted like this:

```
new Plot(y:blahblah) << nana
%result x
x.set(zoom: [23.4, 23.4], pan:[100,25])
```
That way you can recompute the plot and get the same visualization.
For a table, it might be something like:
```
new TableDisplay(stuff)
```
Then you hide a bunch of columns, and add formatting to some of them.
Maybe you even add a new derived column.
Then you rerun the cells that compute stuff, and you want to see the table again.
You select "save interaction" in the menu and a magic like the above is appended to the cell:
```
%result x
x.set(colReorder: ['m1', 'y1', 'y10'], format:[['m1', exp5].....)
```