diffrepro: query stability statistics

Multiple reports can be analyzed for failing / unstable domain when generating statistics.

Afterwards, this data can be used when generating diffsum. Using options -s to provide stats file, along with either --without-ref-unstable or --without-ref-failing to omit these queries from the report. When both options are used, the report contains only previusly unseen failures, indicating either a change in upstrem, or a potential issue with the resolver.

Closes #6 (closed)