Implementing initial layout functionality

samples/custom-layout.yml

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+frame: 
+  x: 38
+  y: 38
+  width: 750
+  height: 1050
+title:
+  x: 125
+  y: 50
+  width: 625
+  height: 150
+subtitle:
+  x: 125
+  y: 150
+  width: 625
+  height: 150
+icon_left:
+  x: 200
+  y: 250
+  width: 125
+  height: 125
+icon_right:
+  x: 400
+  y: 250
+  width: 125
+  height: 125  

samples/use_layout.rb

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+require 'squib'
+
+Squib::Deck.new(layout: 'custom-layout.yml') do 
+  background color: :white
+
+  # Layouts are YAML files that specify x, y, width, and height coordinates
+  rect layout: :frame
+
+  # You can also override a given layout entry in the command
+  rect layout: :frame, width: 50, height: 50
+
+  # Any command with x-y-width-height options, we can use a custom layout
+  text str: 'The Title', layout: :title
+  png file: 'shiny-purse.png', layout: :icon_left
+  svg file: 'spanner.svg', layout: :icon_right
+
+  # Strings can also be used in layouts
+  text str: 'subtitle', layout: 'subtitle'
+
+  save_png prefix: 'layout_'
+end