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").