Custom colors, closing #1

samples/custom-config.yml

@@ -1,3 +1,5 @@
 dpi: 300
 progress_bars: true
-text_hint: '#FF0000'
+text_hint: '#FF0000'
+custom_colors:
+  foo: '#ccc'

samples/custom_config.rb

@@ -2,7 +2,10 @@
 require 'squib'
 
 Squib::Deck.new(config: 'custom-config.yml') do
-	
+
+  # Custom color defined in our config
+  background color: :foo 
+
   # Hints are turned on in the config file
   text str: "The Title", x: 0, y: 78, width: 825,
        font: 'Arial 72', align: :center