sprues: rename "template" to "sprue"

Also:
  * Move stuff around to a builtin/ folder
  * Separate out classes to follow my one-class-per-file convention
  * Samples are renamed to the underscore convention

I just like the word "sprue", and it's more descriptive than our original choice of "template". If I had to start Squib all over again, I would probably name it "Sprue" :P

samples/sprues/_fold_sheet.rb

@@ -6,5 +6,6 @@ Squib::Deck.new(width: '63mm', height: '88mm', cards: 8) do
     str: %w[Front_1 Front_2 Front_3 Front_4 Back_1 Back_2 Back_3 Back_4],
     x: '3mm', y: '3mm'
   )
-  save_pdf file: 'fold_sheet.pdf', template_file: 'templates/fold_sheet.yml'
+  save_pdf file: 'fold_sheet.pdf',
+           sprue: 'my_sprues/fold_sheet.yml'
 end

samples/sprues/_hex_tiles.rb

@@ -10,5 +10,6 @@ Squib::Deck.new(width: '65.8mm', height: '76mm', cards: 9) do
     x: '27mm', y: '35mm', width: '11.8mm', height: '6mm',
     align: :center, valign: :middle
   )
-  save_pdf file: 'hex_tiles.pdf', template_file: 'templates/hex_tiles.yml'
+  save_pdf file: 'hex_tiles.pdf',
+           sprue: 'my_sprues/hex_tiles.yml'
 end

samples/sprues/_use_package_template.rb

@@ -4,5 +4,6 @@ Squib::Deck.new(width: '63mm', height: '88mm', cards: 9) do
   text(
     str: %w[One Two Three Four Five Six Seven Eight Nine], x: '3mm', y: '3mm'
   )
-  save_pdf file: 'use_package_tmpl.pdf', template_file: 'a4_poker_card_9up.yml'
+  save_pdf file: 'use_package_tmpl.pdf', 
+           sprue: 'a4_poker_card_9up.yml'
 end