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