add basic feature for gherkin rule/example support

features/rule.feature

@@ -0,0 +1,110 @@
+Feature: Rule keyword
+
+  Scenario: Rule with multiple examples, passing
+    Given a file named "features/highlander.feature" with:
+      """
+      Feature: Highlander
+
+        Rule: There can be only One
+
+          Example: Only One -- More than one alive
+            Given there are 3 ninjas
+            And there are more than one ninja alive
+            When 2 ninjas meet, they will fight
+            Then one ninja dies
+            And there is one ninja less alive
+
+          Example: Only One -- One alive
+            Given there is only 1 ninja alive
+            Then they will live forever
+      """
+    And a file named "features/step_definitions/cucumber_steps.js" with:
+      """
+      import {Given, When, Then} from 'cucumber'
+      import assert from 'assert'
+
+      Given('there are {int} ninjas', function(count) {
+        this.total = count
+      })
+
+      Given('there is only 1 ninja alive', function() {
+        this.living = 1
+      })
+
+      Given('there are more than one ninja alive', function() {
+        this.living = 2
+      })
+
+      When('2 ninjas meet, they will fight', function() {
+        this.deaths = 1
+        this.living = 1
+      })
+
+      Then('one ninja dies', function() {
+        assert.equal(1, this.deaths)
+      })
+
+      Then('there is one ninja less alive', function() {
+        assert.equal(1, this.living)
+      })
+
+      Then('they will live forever', function() {
+
+      })
+      """
+    When I run cucumber-js
+    Then it passes
+
+  Scenario: Rule with multiple examples, failing
+    Given a file named "features/highlander.feature" with:
+      """
+      Feature: Highlander
+
+        Rule: There can be only One
+
+          Example: Only One -- More than one alive
+            Given there are 3 ninjas
+            And there are more than one ninja alive
+            When 2 ninjas meet, they will fight
+            Then one ninja dies
+            And there is one ninja less alive
+
+          Example: Only One -- One alive
+            Given there is only 1 ninja alive
+            Then they will live forever
+      """
+    And a file named "features/step_definitions/cucumber_steps.js" with:
+      """
+      import {Given, When, Then} from 'cucumber'
+      import assert from 'assert'
+
+      Given('there are {int} ninjas', function(count) {
+        this.total = count
+      })
+
+      Given('there is only 1 ninja alive', function() {
+        this.living = 1
+      })
+
+      Given('there are more than one ninja alive', function() {
+        this.living = 2
+      })
+
+      When('2 ninjas meet, they will fight', function() {
+        // broken implementation...
+      })
+
+      Then('one ninja dies', function() {
+        assert.equal(1, this.deaths)
+      })
+
+      Then('there is one ninja less alive', function() {
+        assert.equal(1, this.living)
+      })
+
+      Then('they will live forever', function() {
+
+      })
+      """
+    When I run cucumber-js
+    Then it fails