Support binary downloads that are not listings

[mist64]

Issue 6/84 has two binary example data files on the Servicediskette that are not printed in the magazine:

• "demo.seq" for "lehrerkalender"
• "supervoc.demo.org" for "supervoc"

We need a way to link to such a file in the HTML and include it in the downloads. Preferably as a <div> or similar that doesn't produce anything visible in the article, with the metadata (i.e. the name of the .prg/.seq file) in an attribute, so that it can be added to the article after the listings.