Support enabled flag for rules.

If a rule has a enabled attribute, and if the value is false, call the
engine's enable_rule() method to disable the rule. Like add_filter,
there's a static method which takes the object as the first argument and
a non-static method that calls the engine.

This fixes #72.

userspace/engine/lua/rule_loader.lua

@@ -188,6 +188,15 @@ function load_rules(rules_content, rules_mgr, verbose, all_events)
 	    else
 	       state.filter_ast = { type = "BinaryBoolOp", operator = "or", left = state.filter_ast, right = filter_ast.filter.value }
 	    end
+
+	    -- Enable/disable the rule
+	    if (v['enabled'] == nil) then
+	       v['enabled'] = true
+	    end
+
+	    if (v['enabled'] == false) then
+	       falco_rules.enable_rule(rules_mgr, v['rule'], 0)
+	    end
 	 else
 	    error ("Unexpected type in load_rule: "..filter_ast.type)
 	 end