Add Epson test page: text sizes, EAN-13, QR, and a raster image

ReceiptBuilder#image implements GS v 0 raster printing (no image library
needed for the demo -- a procedural 8px checkerboard proves it end to
end). Skipped attempting the printer's real-time clock: unsure the
TM-T20II actually has a battery-backed RTC (that's usually an option on
higher-end models), not worth guessing at an undocumented command.

lib/receipt_builder.rb

@@ -93,6 +93,16 @@ class ReceiptBuilder
     self
   end
 
+  # Raster bit image (GS v 0) -- bytes must already be packed 1bpp,
+  # row-major, (width_px/8 rounded up) bytes per row, MSB-first, 1=black.
+  def image(width_px, height_px, bytes)
+    width_bytes = (width_px + 7) / 8
+    @buffer << "\x1D\x76\x30\x00"
+    @buffer << [width_bytes % 256, width_bytes / 256, height_px % 256, height_px / 256].pack('C4')
+    @buffer << bytes
+    self
+  end
+
   def feed(lines = 3)
     @buffer << ("\n" * lines)
     self