reverse-search: add keys for disabling

Now, reverse-search can be disabled with emacs
"Left" and "Right" keys: `Ctrl-B`, `Ctrl-F`.

prompt.go

@@ -227,7 +227,7 @@ func (p *Prompt) feed(b []byte) (shouldExit bool, exec *Exec) {
 				p.buf = newBuf
 			}
 		}
-	case Down, ControlN:
+	case Down, ControlN, ControlB, ControlF:
 		if p.inReverseSearchMode() {
 			p.disableReverseSearch()
 		} else if !completing { // Don't use p.completion.Completing() because it takes double