Add `--keep` flag to `crf-search` subcommand

[t-nil]

For me it's very useful to look at the generated samples for manual verification. This command works on my machine right now.

@alexheretic please tell me if you want the flag refactored into args::Sample. Its probably better that way, right?

[t-nil]

Btw: what is the double purpose of cleaning in main() and sample_encode::run

[alexheretic]

Thanks, I 'm ok with crf-search & auto-encode having --keep. It does make we wonder if we should improve the output a bit though, e.g. informing which temp-dir is being kept so users don't have to guess / set one explicitly.

please tell me if you want the flag refactored into args::Sample. Its probably better that way, right?

Yes I think that makes sense.

Btw: what is the double purpose of cleaning in main() and sample_encode::run

Early cleaning keeps storage usage ceiling down. Cleaning at the end is required to handle ctrl-c interrupts & anything left over.

[t-nil]

Should be fine now :)

[alexheretic]

Looks good now, just one minor code comment.

Can you pop something in the changelog?

[alexheretic]

Thanks!