Add author field

[niksilver]

Thank you for creating this app! I hope you accept this pull request, which I would find useful, and I hope others do, too.

This change adds the <dc:creator> field to show the toot's author, according to the official RSS recommendations.

Background: I'm using mastodon-bookmark-rss to send Mastodon bookmarks to Evernote using IFTTT, which is able to read RSS feeds.

I have tested this locally, and with IFTTT.

Thanks.

[untitaker]

thank you, code looks reasonable to me, just going to deploy it and see if it works :)

[niksilver]

Thank you very much, that's wonderful.