mysql replication: let me specify how bad it is not to be a slave

Choices are all the exit methods: ok, warning, critical or unknown.
Default is ok, so this is 100% backwards compatible
I would take a look to https://github.com/sensu/sensu-plugin/blob/master/lib/sensu-plugin.rb
where parent of every check defines statuses we support. And get check passed attr to this hash instead.

In the same time sending specific method to a object via send is sometimes questionable, but will achieve what you wanted just fine.