Provide option to expand threshold for reading whole file into CRC32 hash

[agirardeau]

When calculating a file hash, files that are <= 16KiB are read in their entirety into a CRC32 fingerprint, while files > 16KiB have only their "crucial bytes" read, as explained in #1 (comment). I work with some files that are in the 50-100KiB range, and are particularly likely to have edits that don't change the file size or affect the critical bytes (they're kinda like csv but with space-padded cells, similar to markdown or org-mode tables). It would be great to just bump up the threshold for fingerprinting the whole file to like 200KiB. I'm using rsync-sidekick for archive so safety is more important than performance. Would it be possible to add an option for this?