Download button: support multimedia

[lukavdplas]

Makes it easier to download multimedia content without converting it to a binary array yourself.

For example, something like this:

using Images
img = load(download("https://whatever"))
DownloadButton(img, "image.png")

Also adds a simple unit test for the download button.

[github-actions[bot]]

Try this Pull Request!

Open Julia and type:

  julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/lukavdplas/PlutoUI.jl", rev="download-button-everything")
julia> using PlutoUI

Or run this code in your browser: