def severity( level, proc = nil, &block )
      proc ||= block
      configuration[:severities][ level.to_s.upcase ] = proc
      self
    end
def severity( level, proc = nil, &block )
      proc ||= block
      configuration[:severities][ level.to_s.upcase ] = proc
      self
    end
Provide default formatting for messages of the given severity when
 a {#match} is not found.

 @overload severity( level, proc )
   @param [String,Symbol] level to format.
   @param [#call(msg)] proc that receives the message and returns the
     reformatted message.
 @overload severity( level )
   @param [String,Symbol] level to format.
   @yieldparam msg [String] the message to reformat.
   @yieldreturn [String] the reformatted message.

 @return [Formatter] self.

 @example
   configure do
     severity(:fatal){ |msg| msg.white_on_red }
   end