Add stderr to cell output

[jonas-schulze]

If a cell causes a warning, I would like to have that included in a rendered Markdown document. How would I achieve this?

function maywarn()
    @warn "whoopsie"
    42
end

maywarn()

Which renders as:

````
42
````

Expected result:

````
┌ Warning: whoopsie
└ @ Main anonymized_path.jl:2
````

````
42
````

Would you be interested in a PR that adds a configuration switch to enable this?

[jonas-schulze]

Having had a look at the code,

https://github.com/fredrikekre/Literate.jl/blob/0d308e4185c322092f0593765ae17acc7c8897a0/src/Literate.jl#L719-L722

I can come close to my desired solution by separating the logging part from the displaying part:

function maywarn()
    @warn "method did not converge"
    42
end

res = maywarn();

#-

res #hide

Which renders as follows. The only downsides are the empty line in the first output fence (see #270) and the two consecutive empty line in between the output fences (should only be one):

````
┌ Warning: method did not converge
└ @ Main.var"##225" ~/tmp/julia-literate-print/warn2.md:2

````


````
42
````