Add warning to suppress png scaling warning

Close #120

samples/load_images.rb

@@ -1,6 +1,6 @@
 require 'squib'
 
-Squib::Deck.new(width: 825, height: 1125, cards: 1) do
+Squib::Deck.new(width: 825, height: 1125, cards: 1, config: 'load_images_config.yml') do
   background color: '#0b7c8e'
   rect x: 38, y: 38, width: 750, height: 1050, x_radius: 38, y_radius: 38
 
@@ -10,7 +10,7 @@ Squib::Deck.new(width: 825, height: 1125, cards: 1) do
   # Can be scaled if width and height are set
   svg file: 'spanner.svg', x: 50, y: 50, width: 250, height: 250
   png file: 'shiny-purse.png', x: 305, y: 50, width: 250, height: 250
-  #...but PNGs will warn if it's an upscale
+  #...but PNGs will warn if it's an upscale, unless you disable them in config.yml
 
   # Can be scaled using just width or height, if one of them is set to :scale
   svg file: 'spanner.svg', x: 200, y: 350, width: 35,     height: :scale

samples/load_images_config.yml

@@ -0,0 +1 @@
+warn_png_scale: false