feature: add use_layout method to load layouts at runtime

Closes #141

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