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!

lib/squib/deck.rb

@@ -49,8 +49,7 @@ module Squib
     # @param height: [Integer] the height of each card in pixels
     # @param cards: [Integer] the number of cards in the deck
     # @param dpi: [Integer] the pixels per inch when rendering out to PDF or calculating using inches. 
-    # @param config: [String] the Yaml file used for global settings of this deck
-    # @param layout: [String] the Yaml file used for layouts.
+    # @param config: [String] the file used for global settings of this deck
     # @param block [Block] the main body of the script.
     # @api public
     def initialize(width: 825, height: 1125, cards: 1, dpi: 300, config: 'config.yml', layout: nil, &block)
@@ -63,7 +62,7 @@ module Squib
       @progress_bar = Squib::Progress.new(false)
       cards.times{ @cards << Squib::Card.new(self, width, height) }
       load_config(config)
-      @layout = YAML.load_file(layout) unless layout.nil?
+      load_layout(layout)
       if block_given?
         instance_eval(&block)
       end
@@ -101,6 +100,60 @@ module Squib
       end
     end
 
+    # Load the layout configuration file, if exists
+    # @api private
+    def load_layout(file)
+      return if file.nil?
+      @layout = {}
+      yml = YAML.load_file(file)
+      yml.each do |key, value|
+        @layout[key] = recurse_extends(yml, key, {})
+      end
+    end
+
+    # Process the extends recursively
+    # :nodoc:
+    # @api private
+    def recurse_extends(yml, key, visited )
+      assert_not_visited(key, visited)
+      return yml[key] unless has_extends?(yml, key)
+      visited[key] = key
+      parent_keys = [yml[key]['extends']].flatten
+      h = {}
+      parent_keys.each do |parent_key|
+        from_extends = yml[key].merge(recurse_extends(yml, parent_key, visited)) do |key, child_val, parent_val|
+          if child_val.to_s.strip.start_with?('+=')
+            parent_val + child_val.sub("+=",'').strip.to_f
+          elsif child_val.to_s.strip.start_with?('-=')
+            parent_val - child_val.sub("-=",'').strip.to_f
+          else 
+            child_val #child overrides parent when merging, no +=
+          end
+        end 
+        h = h.merge(from_extends) do |key, older_sibling, younger_sibling|
+          younger_sibling #when two siblings have the same entry, the "younger" (lower one) overrides
+        end
+      end
+      return h
+    end
+
+    # Does this layout entry have an extends field?
+    # i.e. is it a base-case or will it need recursion?
+    # :nodoc:
+    # @api private
+    def has_extends?(yml, key)
+      yml[key].key?('extends')
+    end
+
+    # Safeguard against malformed circular extends
+    # :nodoc:
+    # @api private
+    def assert_not_visited(key, visited)
+      if visited.key? key
+        raise "Invalid layout: circular extends with '#{key}'"
+      end
+    end
+
     ##################
     ### PUBLIC API ###
     ##################