feature: add use_layout method to load layouts at runtime

Closes #141

docs/dsl/use_layout.rst

@@ -0,0 +1,16 @@
+use_layout
+==========
+
+Load a layout file and merge into the current set of layouts.
+
+Options
+-------
+
+file
+  default: ``'layout.yml'``
+
+  The file or array of files to load. Treated exactly how :doc:`/dsl/deck` parses it.
+
+
+Examples
+--------

lib/squib/api/settings.rb

@@ -31,5 +31,11 @@ module Squib
       @font = (opts[:font] == :default) ? Squib::DEFAULT_FONT: opts[:font]
     end
 
+    # Load a new layout into the deck
+    # @api public
+    def use_layout(file: 'layout.yml')
+      @layout = LayoutParser.load_layout(file, @layout)
+    end
+
   end
 end

lib/squib/layout_parser.rb

@@ -7,8 +7,8 @@ module Squib
 
     # Load the layout file(s), if exists
     # @api private
-    def self.load_layout(files)
-      layout = {}
+    def self.load_layout(files, initial = {})
+      layout = initial
       Squib::logger.info { "  using layout(s): #{files}" }
       Array(files).each do |file|
         thefile = file

samples/layouts.rb

@@ -60,3 +60,13 @@ Squib::Deck.new(layout: 'hand.yml') do
   png file: 'pokercard.png', alpha: 0.5
   save_png prefix: 'layout_builtin_hand_'
 end
+
+# Layouts can also be specified in their own DSL method call
+# Each layout call will progressively be merged with the priors
+Squib::Deck.new do
+  use_layout file: 'custom-layout.yml'
+  use_layout file: 'custom-layout2.yml'
+  text str: 'The Title',   layout: :title     # from custom-layout.yml
+  text str: 'The Subtitle',layout: :subtitle  # redefined in custom-layout2.yml
+  save_png prefix: 'layout3_'
+end

spec/layout_parser_spec.rb

@@ -186,4 +186,16 @@ describe Squib::LayoutParser do
       })
   end
 
+  it 'loads progressively on multiple calls' do
+    a = layout_file('multifile-a.yml')
+    b = layout_file('multifile-b.yml')
+    layout = Squib::LayoutParser.load_layout(a)
+    layout = Squib::LayoutParser.load_layout(b, layout)
+    expect(layout).to eq({
+        'title'    => { 'x' => 300 },
+        'subtitle' => { 'x' => 200 },
+        'desc'     => { 'x' => 400 }
+        })
+  end
+
 end