Take into account additional headers in cp and sync commands.

[bladealslayer]

Support for condition headers.

It is useful for the cp/sync command to support the extra headers, because conditions such as x-amz-copy-source-if-modified-since can be given. PreconditionFailed errors need to be accounted for in responses, as these are normal when conditions are specified.

Example usage:

s3cmd cp -r -v --add-header "x-amz-copy-source-if-modified-since:date -u -d '1 hour ago' | cut -d\ -f 1-4,6" s3://source-bucket/ s3://dest-bucket/