Prevent fingerprints loss in know_hosts

Currently, when adding new fingerprints the whole know_hosts file is rewritten from the loaded fingerprints.
But only fingerprints with a compatible algorithm (from jsch point of view) are kept: for example, all "ecdsa-sha2-nistp256" are lost!

Hence this pull request that appends the new fingerprint in the known_hosts file (without modifying the rest of the file) and keeps unknown fingerprints with unknown algorithms.