def severity_tag( level, proc = nil, &block )
      proc ||= block
      configuration[:severity_tags][ level.to_s.upcase ] = proc
      self
    end
def severity_tag( level, proc = nil, &block )
      proc ||= block
      configuration[:severity_tags][ level.to_s.upcase ] = proc
      self
    end
Format the severity indicator tagged before each line. To format the
 actual message itself use {#severity}.

 @overload severity_tag( level, proc )
   @param [String,Symbol] level to format.
   @param [#call(level)] proc that receives the log level and returns the
     reformatted level.

 @overload severity_tag( level )
   @param [String,Symbol] level to format.
   @yieldparam level [String] the log level to reformat.
   @yieldreturn [String] the reformatted level.

 @return [Formatter] self.

 @example
   configure do
     severity_tag(:warn){|level| level.yellow }
     severity_tag(:error){|level| level.red }
   end