Allow excluding files from cloud syncs

I added an 'exclude' method to Backup::Syncer::Cloud::Base, so excluding files and directories works similarly as in archives:

```ruby
sync_with S3 do |s3|
  s3.directories do |dir|
    dir.add "path/to/directory"
    dir.exclude "path/to/directory/ignored_file.txt"
  end
end
```