Allow parsing of multiple authors with email addresses

Arcdown currently expects each article to have only a single author, but there is currently no functionality in place for handling multiple authors per article. We should add parsing for the possibility of multiple authors per article, comma-separated as a list, like this:

`by Julian Blaustein <julian@blaustein.com>, Jules Verne <jules@verne.net>, H.G. Wells <hg@wells.org>`