Split media URLs at w-1 chars, not w.

Split media URLs at w-1 chars, not w.

This makes no real difference, but now they match the URLs in the text
of posts. In the Python version both split at w, but now that I'm
delegating HTML processing to html2text, that splits overlong words at
the same width it wraps sensible lines to. So I have to either have
them inconsistent or change the media one.

w-1 is more 'Mono' anyway, let's be honest :-)