implement placeholder functionality

needs testing but almost there

samples/images/_placeholders.rb

@@ -0,0 +1,28 @@
+# require 'squib'
+require_relative '../../lib/squib'
+
+# By default Squib will simply warn you if an image is missing
+# Instead, you can give it a `placeholder`
+Squib.configure img_missing: :silent  # no warnings, no errors, no placeholder
+# Squib.configure img_missing: :warn  # default
+# Squib.configure img_missing: :error # pre Squib v0.18 behavior... blech
+
+Squib::Deck.new(width: 100, height: 100, cards: 3) do
+  background color: :white
+
+  files = %w(angler-fish.png does-not-exist.png) # last one is nil
+  png file: files, placeholder: 'grit.png'
+  save_sheet columns: 1, prefix: 'placeholder_sheet_'
+end
+
+# Placeholders can be per-image too.
+# What if a placeholder is specified but doesn't exist?
+# It'll always warn.
+Squib::Deck.new(width: 100, height: 100, cards: 3) do
+  background color: :white
+
+  files =        %w(angler-fish.png does-not-exist.png does-not-exist.png)
+  placeholders = %w(grit.png        does-not-exist.png grit.png)
+  png file: files, placeholder: placeholders
+  save_sheet columns: 1, prefix: 'multi_placeholder_sheet_'
+end