Added support for merging multiple layout files

spec/data/multifile-a.yml

@@ -0,0 +1,4 @@
+title:
+  x: 100
+subtitle:
+  x: 200

spec/data/multifile-b.yml

@@ -0,0 +1,4 @@
+title: # redefined
+  x: 300
+desc:
+  x: 400

spec/data/multifile-extends-a.yml

@@ -0,0 +1,8 @@
+grandparent:
+  x: 100
+parent_a:
+  extends: grandparent
+  x: += 10   # evaluates to 110
+parent_b: 
+  extends: grandparent
+  x: += 20   # evaluates to 120, for now...

spec/data/multifile-extends-b.yml

@@ -0,0 +1,9 @@
+child_a:
+  extends: parent_a
+  x: += 3    # evaluates to 113
+parent_b:    # redefined
+  extends: grandparent
+  x: += 30   # evaluates to 130
+child_b:
+  extends: parent_b
+  x: += 3    # evaluates to 133

spec/deck_spec.rb

@@ -183,6 +183,30 @@ describe Squib::Deck do
         raise_error(RuntimeError, 'Invalid layout: circular extends with \'a\'')
     end
 
+    it 'redefines keys on multiple layouts' do
+      a = test_file('multifile-a.yml')
+      b = test_file('multifile-b.yml')
+      d = Squib::Deck.new(layout: [a, b])
+      expect(d.layout).to eq({
+          'title'    => { 'x' => 300 },
+          'subtitle' => { 'x' => 200 }, 
+          'desc'     => { 'x' => 400 }
+          })
+    end
+
+    it 'evaluates extends on each file first' do
+      a = test_file('multifile-extends-a.yml')
+      b = test_file('multifile-extends-b.yml')
+      d = Squib::Deck.new(layout: [a, b])
+      expect(d.layout).to eq({
+          'grandparent' => { 'x' => 100 },
+          'parent_a'    => { 'x' => 110, 'extends' => 'grandparent' }, 
+          'parent_b'    => { 'x' => 130, 'extends' => 'grandparent' },
+          'child_a'     => { 'x' => 113, 'extends' => 'parent_a' },
+          'child_b'     => { 'x' => 133, 'extends' => 'parent_b' }
+          })
+    end
+
   end
 
 end