allow embeds from itch.io

[kareila]

https://www.dreamwidth.org/support/see_request?id=47796

Provided examples:

<iframe height="167" frameborder="0" src="https://itch.io/embed/1731661" width="552"><a href="https://zephyo.itch.io/an-average-day-at-the-cat-cafe">an average day at the cat cafe by angela he, Alex Rose, atmospherium</a></iframe>

<iframe height="167" frameborder="0" src="https://itch.io/embed/1744815" width="552"><a href="https://jetamp.itch.io/boba-simulator">Boba Simulator by Napas</a></iframe>