Implement `layout' run control keyword

Allow configuration of the layout by specifying either a class or an
instance.

features/run_control/layout.feature

@@ -0,0 +1,27 @@
+Feature: `layout' run control keyword
+
+  Background:
+    Given a file named my_layout.rb with:
+      """
+      class MyLayout
+        def register *_
+          puts "testing_rc_layout"
+        end
+      end
+      """
+
+  Scenario: configures a layout class
+    Given a run control file with:
+      """
+      layout MyLayout
+      """
+    When I run uhwm with options -r./my_layout
+    Then the output must contain "testing_rc_layout"
+
+  Scenario: configures a layout instance
+    Given a run control file with:
+      """
+      layout MyLayout.new
+      """
+    When I run uhwm with options -r./my_layout
+    Then the output must contain "testing_rc_layout"