[PROPOSE] rename parsed_time_tag option to output_key

This is a proposition. How about renaming `parsed_time_tag` option to `output_key`?

`parsed_time_tag` option sounds like that it is specifying a tag name, but it is actually a key name of the output record. So, I felt `parsed_time_key`, rather `output_key` is more suitable. 

In addition, I changed the default value of `output_key` to be the value of `key`. This change is because I felt as it is okay to discard (overwrite) the original tai64n field for most of usecases. With this change, we do not need to use fluent-plugin-record-reformer to discard tai64n field as you said in your blog as default. 

> sonots/fluent-plugin-record-reformerのようなプラグインを使って tai64n自体のキーを消して、DBに入れればTAI64N形式のタイムスタンプを二度と見なくて良くなる。

Right, this patch breaks the backward compatibility, but I think now is the only chance :D