Screenshot support

[MrOrz]

From https://g0v.hackmd.io/DSuREu46QbWI38ITq9IlrQ#%E4%BF%AE%E5%BE%A9%E5%A4%B1%E6%95%88%E9%80%A3%E7%B5%90
we can consider adding screenshot functionality so that when the URL goes down, we can still access what it looked like previously.