Add --filter-not-empty option

[fpytloun]

It would be great to have option to skip empty files.

Use-case is to skip files that are eventually opened by another process and not flushed and closed yet.

Might be also possible to solve this by constructing list of files to upload with find and pass the list to s3sync, eventually pass list of excluded files. But that is not supported as Syncing of single file are not supported

This use-case might be also solved by adding --filter-closed or --filter-not-opened that would check if file is opened by another process and skip such files from sync.