Contrast message from the level

Using this syntax:

require 'yell'
z = Yell.new(STDOUT, :colors => true)
z.warn 'abcde'

I get colored output, but the level and message are colored the same. I think it
would be better to contrast the message from the level. Some projects from
Python color the level only:

from logzero import logger
logger.warning('abcde')

or color the message only:

import coloredlogs, logging
coloredlogs.install()
logging.warning('abcde')