add %{..} placeholder syntax

Since fluent-plugin-s3 uses the %{..} syntax on s3_object_key_format, it would be cleaner to support this placeholder so hostname, uuid, etc. can be used by mixing-in this gem rather than re-implementing these features independently again.

Reference:
https://github.com/fluent/fluent-plugin-s3/pull/16
https://github.com/ijin/fluent-plugin-s3/commit/aa5a9624ae7467a7e8f838e7fab1052b38bec56c