Document how to modify IOContext parameters in outputs

[mortenpi]

I.e. number of columns of the REPL output etc. -- would be good to have a short how-to guide for this in the docs. Related issues:

• Ability to set IOContext in which @example/@repl blocks are run #1450
• Controlling output text width in doctests #942

@fredrikekre: in reference to #1450, how do you imagine the user using DisplayAs.(set|with)context in Documenter? Explicitly call it in the at-example/repl block? I.e. something like this?

```@example
f() = collect('a':'z') # hide
f() |> DisplayAs.withcontext(...)
```

[asinghvi17]

It would be great to have these as "keyword arguments" to the example/repl blocks as well - as well as maybe some defaults on the page level, settable via @meta. That way there is less of a barrier to entry - and people can easily opt in, versus googling to get to DisplayAs and then installing the package, making sure it's used (and hidden) in every example environment, etc.

[mortenpi]

Yes, they should be keywords. We also got JuliaDocs/IOCapture.jl#26 now, so it should be possible to pass these on pretty easily.

One first step towards all that would be to harmonize the parsing on the at-block arguments. I would like there to be a single function that you pass the "language" attribute string (i.e. @example foo; x = 2, y = 3) and it returns a data structure that has the parsed arguments.