Need a `cleanup` mode for cleaning up orphans

[cr]

Over time the local cache accumulates web extension files that are no longer reference by the latest AMO metadata. In the long run we might consider to amend instead of replace the metadata to build up a historic metadata record, but this is likely a rather complicated and brittle process. Until then, issuing webextware cleanup [--really] to list and purge all the orphans would come in handy.

[cr]

Afterthought: It might as well come as a --purge option to sync, because there are already other ways to list orphans (webextaware query orphans).