support reading older WARC versions

The reader code barfs on versions other than "WARC/1.0".

I have not seen anything on what are the differences between, say, 1.0, 0.18 and 0.17 (apart from the version stamp itself). If version 1.0 is otherwise equal to either or both of those, please allow reading them, or add a configuration variable that determines whether they are alllowed.

I could fork the code and add this feature, but I do not know of the differences. If someone can point me to spec on 1.0, I'd be happy to do it.