support for updating article metadata

Here's another PR which gets update_article() working.  No CLI for this -- I'm not sure how best to implement anyway with all the args -- but it works nicely when called in another script.

(By the way, my use-case is uploading data that has been split into many <250 MB files, and I really didn't want to manually edit the metadata on every article individually.  Thanks for writing this, and especially for taking care of all the annoying authentication steps!)