$newsrc->unmarked_articles(groupname, n, m[, options])
Returns the list of articles from n to m, inclusive, that are not
in the article list for the newsgroup. Arguments are:
- groupname
Group to return articles from.
- n
Number of the first article in the range.
- m
Number of the last article in the range.
- options
Hash. See description under add_group.
