vi bindings: add CTRL-C handler in 'default' mode

There is no CTRL-C handler for the default mode in the vi bindings. This makes it difficult to say "never mind" and start a new command line like you can do in bash's vi mode. 

There were CTRL-C handlers for insert and visual modes that go back to default mode, but nothing happens in default mode. I copy-pasted the CTRL-C handler from the default key bindings file.