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!

samples/custom-layout.yml

@@ -18,22 +18,42 @@ subtitle:
   height: 60
   align: !ruby/symbol center
   valign: !ruby/symbol middle
-
-# Yaml has this beautfiul feature for us: merge keys
-# http://www.yaml.org/YAML_for_ruby.html#merge_key
-# Define an alias with &foo, then use <<: *foo to include it
-# Everything gets merged, with later merges overriding 
-icon: &icon
+icon:
   width: 125
   height: 125
   y: 250
 icon_left:
-  <<: *icon 
+  extends: icon
   x: 150
 icon_middle:
-  <<: *icon
+  extends: icon
   x: 350
   y: 400  #overrides the y inherited from icon
 icon_right:
-  <<: *icon
+  extends: icon
   x: 550
+
+# Squib also supports its own merging-and-modify feature
+# Called "extends"
+# Any layout can extend another layout, so long as it's not a circle
+# Order doesn't matter since it's done after YAML procesing
+# And, if the entry overrides 
+bonus: #becomes our bonus rectangle
+  x: 250
+  y: 600
+  width:  300
+  height: 200
+  radius: 32
+bonus_inner: 
+  extends: bonus 
+  x: += 10 # i.e. 260
+  y: += 10 # i.e. 610
+  width: -= 20  # i.e. 180
+  height: -= 20 # i.e. 180
+  radius: -= 8
+bonus_text:
+  extends: bonus_inner
+  x: +=10
+  y: +=10
+  width: -= 20
+  height: -= 20

samples/use_layout.rb

@@ -13,17 +13,22 @@ Squib::Deck.new(layout: 'custom-layout.yml') do
   # Lots of commands have the :layout option
   text str: 'The Title', layout: :title
 
-  # Layouts also support an "extends" attribute to reuse settings
+  # Layouts also support YAML merge keys toreuse settings
   svg file: 'spanner.svg', layout: :icon_left
   png file: 'shiny-purse.png', layout: :icon_middle
   svg file: 'spanner.svg', layout: :icon_right
 
+  # Squib has its own, richer merge key: "extends"
+  rect fill_color: :black, layout: :bonus
+  rect fill_color: :white, layout: :bonus_inner
+  text str: 'Extends!',    layout: :bonus_text
+
   # Strings can also be used to specify a layout (e.g. from a data file)
   text str: 'subtitle', layout: 'subtitle'
 
   # For debugging purposes, you can always print out the loaded layout
   #require 'pp'
   #pp @layout
-
+  
   save_png prefix: 'layout_'
 end