feat: Allow --todo argument.

tests/.snapshots/todos.txt

@@ -0,0 +1,31 @@
+   TODO  Tests\Features\BeforeEachProxiesToTestCallWithTodo - 4 todos
+  ↓ is marked as todo 1
+  ↓ is marked as todo 2
+  ↓ is marked as todo 3
+  ↓ shouldBeMarkedAsTodo
+
+   TODO  Tests\Features\DatasetsTests - 1 todo
+  ↓ forbids to define tests in Datasets dirs and Datasets.php files
+
+   TODO  Tests\Features\Describe - 5 todos
+  ↓ todo
+  ↓ todo on hook → should not fail
+  ↓ todo on hook → should run
+  ↓ todo on describe → should not fail
+  ↓ todo on describe → should run
+
+   TODO  Tests\Features\Todo - 3 todos
+  ↓ something todo later
+  ↓ something todo later chained
+  ↓ something todo later chained and with function body
+
+   PASS  Tests\CustomTestCase\ChildTest
+  ✓ override method
+
+   PASS  Tests\CustomTestCase\ExecutedTest
+  ✓ that gets executed
+
+   PASS  Tests\CustomTestCase\ParentTest
+  ✓ override method
+
+  Tests:    13 todos, 3 passed (3 assertions)