Add support for fetching Atom content src URL

[boehs]

in my atom feeds I do not send the full content for all 40 entries. instead, I do <content src="..."/> as described in the Atom spec.

   atom:content MAY have a "src" attribute, whose value MUST be an IRI
   reference [RFC3987].  If the "src" attribute is present, atom:content
   MUST be empty.  Atom Processors MAY use the IRI to retrieve the
   content and MAY choose to ignore remote content or to present it in a
   different manner than local content.

Atom Processors MAY use the IRI to retrieve the content

If this feature were to be supported, essentially the "download" button would be pressed automatically for <content src=""/>

Also, this exact behaviour isn't defined in the spec, but if there is a link rel alternate and <content> is otherwise non-existent, the alternate could be presumably fetched

atom:entry elements that contain no child atom:content element MUST contain at least one atom:link element with a rel attribute value of "alternate".