ARD Audiothek importer

[kellnerd]

Another unreleased userscript, already working but still has a few rough edges (see commit message).

It's been two years and I haven't used it since, so I wonder whether it is still worth to be released? The branch needs to be rebased in any case and the refactoring probably conflicts with recent changes of the DRA importer from #43.

I might reimplement this importer as a Harmony provider one day.