request: add estimated overlapping bp to output

[ctb]

Right now the only numerical output column is containment, which is relative to the query.

If you're doing larger queries (e.g. for detecting host contamination) and/or queries with many different sized genomes, it would be nice to be able to also have the number of hashes somewhere. In sourmash gather we output this x scaled as intersect_bp.

It would also be good to be able to threshold matches on this number rather than on containment, for the same reasons!