Cleanup of img_dir and testing of input_file.rb

samples/custom-config.yml

@@ -2,4 +2,4 @@ progress_bars: true
 text_hint: '#FF0000'
 custom_colors:
   foo: '#ccc'
-img_dir: customconfig-imgdir
+img_dir: customconfig-imgdir

samples/custom_config.rb

@@ -9,7 +9,7 @@ Squib::Deck.new(config: 'custom-config.yml') do
        font: 'Arial 72', align: :center
 
   # Progress bars are shown for these commands
-  # And images are taken from config-imgdir
+  # And images are taken from img_dir, not the cwd.
   png file: 'shiny-purse2.png', x: 620, y: 75
   svg file: 'spanner2.svg', x: 620, y: 218
   save_png prefix: 'custom-config_'