Warn when closing without saving

[aDotInTheVoid]

Currently, you can run ^c and exit with unsaved work, and xi-term will exit without warning.

Ideally, the "normal" exit should not work with unsaved text. I think vim handles this nicely, where one usually exits with :q, but this does not work with unsaved work. Instead, vim warns you of this but also lets you use :q! instead.