Support reading akregator feed archive

[apaku]

This should be usable as a kind of importer of data so one can get existing feeds and all the articles out of akregator into slimfeed.

[apaku]

Turns out akregator brings a commandline utility akregatorstorageexporter which merely needs to be called with the url. So along with the feeds.opml file from its data-dir might be enough already.