Controlling output text width in doctests

[mortenpi]

Would be nice to be able control the width of the doctest outputs (e.g. whether / when the matrix printing routines should start omitting rows and columns). It should be possible to pass some jldoctest options as IOContext.

[nickrobinson251]

I set this for all docs by setting the ENV["COLUMNS"] variable to something >80 in docs/make.jl. See ? displaysize for how that's used.