Support saving plaintext animations in 'asciicast' file format

[trevorld]

• Already an R package (not on CRAN) with code to save in such a format: https://github.com/coolbutuseless/asciicast
• But the spec file format is pretty simple: https://github.com/asciinema/asciinema/blob/master/doc/asciicast-v2.md
• Support in cat_game() or animate_game()?