Add support for kitty's terminal protocol

[mrzv]

It seems shpool happily passes on escape sequences to the outer shell, which makes it work great with kitty's graphics protocol. But I guess it doesn't save those escape sequences, and so if I disconnect and reconnect, the images are lost. It probably doesn't make sense to store all escape sequences, but storing specifically those for the graphics protocol would be amazing.

[ethanpailes]

@Aetf might be something to think about in terms of the shpool_vt100 replacement (we should also make an issue to track that overall effort).

[mrzv]

Same comment goes for kitty's Shell integration. It currently works great in the original invocation, but breaks in the restored session. The problem also boils down to restoring certain escape codes.

[ethanpailes]

This is a longer term effort, that will probably take us a while, but I do think it is something we should do. @Aetf's work on #46 will hopefully be able to roll this up, though if we have to do something more than just storing the control codes for replaying it might not be in the first version of the vt100 replacement.