Support setting the log level.

[kill9zombie]

Set the log level either in the constructor or via the 'level' property.

For example:

logger = Syslog::Logger.new(
  facility: Syslog::Facility::LOCAL1,
  level: Syslog::Severity::INFO
)

# .. then optionally change the level later
logger.level = Syslog::Severity::DEBUG

In my scenario we read the log level from a config file, so we might end up changing it after the object is created.

Thanks for the shard, it's just what we needed.