Enable to output expand config as debug log

[syucream]

For convenience :)

output example:

...
2017-09-21 16:36:06 +0900 [debug]: #0 fluent/log.rb:296:call: [Fluent::Plugin::ConfigExpanderOutput] expand config:
<match>
  @type stdout
  output_type json
</match>

...
2017-09-21 16:36:06 +0900 [debug]: #0 fluent/log.rb:296:call: [Fluent::Plugin::ConfigExpanderInput] expand config:
<source>
  @type forward
</source>