Allow parsing of multiple authors with email addresses

[zazaian]

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 <[email protected]>, Jules Verne <[email protected]>, H.G. Wells <[email protected]>