Add log level option to subscribe method

[jsvd]

The idea behind this is to allow

logger = Cabin::Channel.get
logger.level = :info
logger.subscribe(File.open("/tmp/log", "a"), :info) # :debug would be the same as nil
logger.subscribe(STDOUT, :warn)

Such that:

• logger.debug("test") won't be sent anywhere
• logger.info("test") is sent only to the logfile
• logger.fatal("test") is sent to both

[jordansissel]

hmm, I'm open to this idea. For backwards compat, we can have subscribe(IO) with no level default to :debug