support for hostname, uuid, etc. from fluent-mixin-config-placeholders

This change supports additional variables from fluent-mixin-config-placeholders such as %{hostname} and %{uuid} in s3_object_key_format

<pre>
s3_object_key_format %{path}%{time_slice}_%{index}-%{hostname}.%{file_extension}
</pre>

Original thread:
https://github.com/fluent/fluent-plugin-s3/pull/16