[exercises/runner.js, exercises/higher_order_functions] Allow setting 'ignoreReturnValue' on exercises to ignore return value of submitted function.

Closes timoxley#128.

Author: timoxley

exercises/higher_order_functions/exercise.js

@@ -11,3 +11,5 @@ var exercise = module.exports = runner.init(function() {
 }).quiet(function count() {
   console.log(exercise.__('call_log', ++counter))
 }, random.int(3, 10))
+
+exercise.ignoreReturnValue = true

exercises/runner.js

@@ -83,7 +83,8 @@ function runner() {
     if (verbose) {
       console.log(__('solution'), util.inspect(solutionResult, { colors: true }).replace(/,\n\s*/g, ", "))
     }
-    callback(null, deepEqual(submittedResult, solutionResult))
+    var resultsMatch = exercise.ignoreReturnValue ? true : deepEqual(submittedResult, solutionResult)
+    callback(null, resultsMatch)
   })
 
   if (wrapUpFx) {