Add custom variables to subscriptions

This PR adds the ability to define custom variables with subscriptions. The use case for this is when I'm using ```sensu::subscription```'s in my various modules to subscribe to classes of checks that are scheduled on the server, but the server might not necessarily be pointing to the same resources as the client and so I'd like to make use of Sensu's ```:::parameter.name:::``` syntax. Trying to set all of the module-specific variables in my one class that installs the base Sensu client becomes messy and polluted with variables that don't belong there so it's tidier if they can be defined with the subscription that will ultimately use them.

I copied the code in the other types and also added some tests, (the sensu_subscription_spec.rb was empty previously so I just based them off the other definition tests).