News::Newsrc manages newsrc files, of the style

    alt.foo: 1-21,28,31-34
    alt.bar! 3,5,9-2900,2902

Methods are provided for
- reading and writing newsrc files
- adding and removing newsgroups
- changing the order of newsgroups
- subscribing and unsubscribing from newsgroups
- testing whether groups exist and are subscribed
- marking and unmarking articles
- testing whether articles are marked
- returning lists of newsgroups
- returning lists of articles
