Configure nested sections

Extend DSL to configure parameters of plugins, to define nested sections with its parameter name/type definitions.
Add proxy object as receiver of `config_*` methods to implement this to configure recursively with config element trees.

* definition example
```ruby
# class GreatNoodleReportPlugin
config_param :name, :string, default: "frsyuki"
config_section :userinfo, required: true, multi: false do # for just 1 <userinfo> section
  config_param :address, :string
  config_param :age, :integer
  config_param :admin, default: false
end
config_section :noodle, param_name: :noodles, required: false, multi: true do # for any times of <noodle NAME> sections
  config_argument :name, :string # section argument NAME required (default option is also available)
  config_param :cost, :integer
  config_section :shop, param_name: 'shops' do # string value is also acceptable
    config_argument :magazine_page do |val|
      raise Fluent::ConfigError, "magazine_page argument is required" if val.nil? || val.empty?
      val.to_i
    end
    config_param :name, :string, default: nil
    config_param :stars, :integer, default: nil
  end
end
```

* configuration example
```apache
<source>
  type great_noodle_report
  name repeatedly
  <userinfo>
    address Marunouchi
    age 99
    admin false
  </userinfo>
  <noodle ramen>
    cost 600
    <shop 50>
      name ten-ichi
      stars 5
    </shop>
    <shop 51>
      stars 1
    </shop>
  </noodle>
  <noodle tsukemen>
    cost 800
    <shop 32>
      stars 800
    </shop>
  </noodle>
</source>
```

* ruby code to use these values
```ruby
# in plugin's instance methods
@name #=> "repeatedly"
@userinfo.age #=> 99

@noodles # Array(size 2), not "noodle" by param_name

@noodles[1].name #=> "tsukemen"
@noodles[1].cost #=> 800
@noodles[1][:cost] #=> 800
@noodles[1].shops # Array(size 1)
@noodles[1].shops.first.stars #=> 800
```

NOTES:
 * Max nest depth is not limited
 * Both of v1 and traditional configuration syntax may be supported
   * This patch does NOT touch internals of Parser, Element or types