Support reference in conditions

[mavarazy]

Following #12 comment, we want to be able to do comparison between fields in the object like this

conditions: {
    a: { less: "$b" }
}

And also support arbitrary predicates:

conditions: {
     cost: {
        decreasedByMoreThanPercent: {
             average: "averages_monthly.cost",
             target: 20
         }
     }
 }