Save sketches for non-sketch commands automatically

[bovee]

Right now when a FASTX file is passed in to dist, etc commands, we sketch the file and keep it entirely in memory for comparisons and then drop the sketch when we exit. Maybe we should automatically save the sketches next to the sequencing files (and disable this saving via a flag) instead? This would speed up repetitive distance comparisons, but would create files that aren't necessarily asked for.