add auto_key settings (without pattern#{i})

[muddydixon]

おそらく、データサイズが無制限に増えるのを抑制するためにpatternを明示的に定義するようにしたとは思うのですが、patternが無いほうが助かるケースがあったので変更しました。

[tagomoris]

I want to merge this request, but items below block:

• Implicit declaration is not good
  • Please add configuration item such as 'count_all_valiation :bool, :default => nil' (this name seemds very bad...)
• No tests exist

[muddydixon]

I add test code, explicit declaration "count_all_patterns" and config validation.

[tagomoris]

I'm sorry to decline this pull request. Unlimited memory usage is very big problem.

The better way is to make another plugin like fluent-plugin-variation-counter or so.