Add screenshot utility

cap.py

+#!/usr/bin/env python3
+import os
+import sys
+from interaction import get_exapunks_window, screenshot_board
+
+
+if __name__ == '__main__':
+    win = get_exapunks_window()
+    #win.raise_window()
+    board = screenshot_board(win).convert('RGB')
+    os.makedirs('screens', exist_ok=True)
+    board.save('screens/board%d.png' % int(sys.argv[1]))