Pluto & PyPlot.jl `imshow`

Hey,

I've seen that `PyPlot.jl` in general works, but in Pluto nothing really happens for:

```julia
begin
using PyPlot
PyPlot.imshow(randn((3,3)))
end
```
Only some output:
```julia
PyObject
```

In the REPL that pops-up an external, interactive viewer. In Jupyter it is within the browser, but I would be happy to have it externally.

Thanks a lot,

Felix

Setup:
* PyPlot v2.9.0
* Pluto v0.15.1

Ref: [Pluto issue](https://github.com/fonsp/Pluto.jl/issues/1422)

Reproduce the Julia snippet in Pluto using PyPlot v2.9.0 and Pluto v0.15.1, then compare the behavior with the linked Pluto issue. Trace how PyPlot's imshow result is handled in Pluto; done means the image is displayed in a usable viewer rather than only returning a PyObject representation.

Vom Indexierungsmodell aus dem Issue-Text verfasst.