engine: exit on Ctrl-D, just like Ctrl-C

Many common CLI applications quit when stdin reaches end-of-file, as
triggered by Ctrl-D. (Examples: `python`, `bash`, `node`, `gdb`, `ed`.)
Though `jiq` is more of a TUI than a CLI, exiting with Ctrl-D still
feels familiar and intuitive to me, and was how I tried to exit `jiq`
the first time that I used it.

wchargin-branch: ctrl-d-exit
wchargin-source: b3ac9be9ce4378b8c3eb82e6d3bc3bb6403d3309

Author: wchargin

engine.go

@@ -168,7 +168,7 @@ func (e *Engine) Run() *EngineResult {
 					}
 				}
 				e.confirmCandidate()
-			case termbox.KeyCtrlC:
+			case termbox.KeyCtrlC, termbox.KeyCtrlD:
 				return &EngineResult{}
 			default:
 			}