Add support for authors in the RSS feed
Adds support for author names in RSS feeds by checking whether an "author" field exist in the item environment, and passing it to OCaml-RSS.
Remark: the RSS specification requires an "email" of the form "foo@bar.com" or "foo@bar.com (Firstname Lastname)" as author, but my stog posts have used the "FirstName LastName" format and I don't think there is a point in adding email information: tested RSS readers accept email-less author information just fine. We also sometimes have several authors ("Ms Foo, Mr Bar").