Enhancement request: add a -k (keep) option like bzip2

[Neil Mayhew]

Sometimes it's useful to be able to keep the original file around. 
Currently, the only way to keep the original file is either to copy it, 
which is wasteful, or to use stdin/stdout redirection, which doesn't 
preserve the modification time and requires the user to give the output 
file name explicitly, which is error-prone and annoying. I assume gzip 
does an unlink of the original file at the end, so the unlink option 
would just request that this not be done.