Bringing back the extends feature

Adding in my own merge key implementation that allows you to bring in
other keys and modfiy the data. Makes moving boxes around easier.

A quasi-revert of 00e8b59144
...but with something useful!

spec/data/easy-circular-extends.yml

@@ -0,0 +1,6 @@
+a: 
+  extends: b
+  x: 50
+b:
+  extends: a
+  x: 150

spec/data/hard-circular-extends.yml

@@ -0,0 +1,9 @@
+a: 
+  extends: c
+  x: 50
+b:
+  extends: a
+  x: 150
+c: 
+  extends: b
+  y: 250

spec/data/multi-extends-single-entry.yml

@@ -1,13 +1,14 @@
-aunt: &aunt
+aunt: 
   a: 101
   b: 102
   c: 103
-uncle: &uncle
+uncle: 
   x: 104
   y: 105
   b: 106
 child:
-  <<: *uncle
-  <<: *aunt
+  extends: 
+    - uncle
+    - aunt
   a: 107
   x: 108

spec/data/multi-level-extends.yml

@@ -1,10 +1,10 @@
-frame: &frame
+frame: 
   x: 38
   y: 38
-title: &title
-  <<: *frame
+title:
+  extends: frame
   y: 50
   width: 100
-subtitle: 
-  <<: *title
+subtitle:
+  extends: title
   y: 150

spec/data/pre-extends.yml

@@ -0,0 +1,7 @@
+title:
+  extends: frame
+  y: 50
+  width: 100
+frame: 
+  x: 38
+  y: 38

spec/data/self-circular-extends.yml

@@ -0,0 +1,3 @@
+a: 
+  extends: a
+  x: 50

spec/data/single-extends.yml

@@ -1,7 +1,7 @@
-frame: &frame
+frame: 
   x: 38
   y: 38
 title:
-  <<: *frame
+  extends: frame
   y: 50
   width: 100

spec/data/single-level-multi-extends.yml

@@ -1,12 +1,12 @@
-frame: &frame
+frame: 
   x: 38
   y: 38
 title:
-  <<: *frame
+  extends: frame
   y: 50
   width: 100
 title2:
-  <<: *frame
+  extends: frame
   x: 75
   y: 150
   width: 150