feature request: limit kmer db construction to kmers in another db

[notestaff]

When constructing a kmer db from reads, sometimes I'm only interested in kmers that appear in another kmer db (e.g. kmers that have appeared in known genomes of taxon of interest). Right now I must first construct a db of all kmers in the reads, and only afterwards intersect that with the (much smaller) db of kmers of interest. It would be better if there was a way to stop uninteresting kmers from being added to the db in the first place. So, in addition to -ci/-cx options for kmc, there'd be an option to give an existing kmer db, and to only add to the newly constructed db kmers that appear in the existing kmer db. @marekkokot