Add deploy support for itch.io

[qreeves]

Use Butler to push builds directly to itch.io from within semdeploy.sh.

From itchio/itch#1609:

I still really recommend using https://itch.io/docs/butler to push builds directly to itch.io so that

• They're served via our CDN
• The app can tell when a new file is available (external URLs never change, so it can't know a new version is available currently)
• It can update the game by applying small patches rather than downloading the whole game.