Add archiving parameter for old versions of articles

[eseiver]

Expanding on @mpacer's thoughts about allowing multiple corpora, create a default setting where when a new version of an article is downloaded, it automatically archives the old version in a different folder. This would probably be best as a sub-directory of corpus.
If a user wanted to go back into the past and get old versions of all articles (e.g. all uncorrected proofs), there would need to be a way to do that as well.