def define_group(name, &block)
      subconfig = self.class.new(self)
      subconfig.filter = name
      subconfig.docs_dir = self.docs_dir.join(name.to_s)
      yield subconfig
      groups << subconfig
    end
def define_group(name, &block)
      subconfig = self.class.new(self)
      subconfig.filter = name
      subconfig.docs_dir = self.docs_dir.join(name.to_s)
      yield subconfig
      groups << subconfig
    end
Defines a new sub configuration

 Automatically sets the `filter` to the group name, and the `docs_dir` to
 a subfolder of the parent's `doc_dir` named the group name.

   RspecApiDocumentation.configure do |config|
     config.docs_dir = "doc/api"
     config.define_group(:public) do |config|
       # Default values
       config.docs_dir = "doc/api/public"
       config.filter = :public
     end
   end

 Params:
 +name+:: String name of the group
 +block+:: Block configuration block