Pipe crashes Vorpal

I have some issues with pipes in [birdknife](https://github.com/vanita5/birdknife).

I am using `.catch` to interpret non-command input as status updates. I had some issues with quote and equal chars before, but I was able to fix this with a custom command parser.

Now, this isn't possible with pipes (`|`). They are interpreted by Vorpal before being passed to the parser, so I am not able to bypass this behavior.
Any chance I can disable the interpretation of pipes? I don't need them at all.

Also, there's a bug: A single pipe as the input crashes the application:

``` bash
birdknife [139]> |

E:\localnet\birdknife\node_modules\vorpal\node_modules\inquirer\node_modules\rx-lite\rx.lite.js:75
throw e;
^
TypeError: Cannot read property 'join' of undefined
at CommandInstance. (E:\localnet\birdknife\birdknife.js:446:32)
at EventEmitter.session.execCommandSet (E:\localnet\birdknife\node_modules\vorpal\dist\session.js:470:20)
at EventEmitter.vorpal._exec (E:\localnet\birdknife\node_modules\vorpal\dist\vorpal.js:956:18)
at EventEmitter.vorpal._execQueueItem (E:\localnet\birdknife\node_modules\vorpal\dist\vorpal.js:747:17)
at EventEmitter.vorpal._queueHandler (E:\localnet\birdknife\node_modules\vorpal\dist\vorpal.js:731:10)
at EventEmitter.vorpal.exec (E:\localnet\birdknife\node_modules\vorpal\dist\vorpal.js:678:10)
at E:\localnet\birdknife\node_modules\vorpal\dist\vorpal.js:621:10
at null.completed (E:\localnet\birdknife\node_modules\vorpal\dist\ui.js:215:13)
at PromptUI.onCompletion (E:\localnet\birdknife\node_modules\vorpal\node_modules\inquirer\lib\ui\prompt.js:57:10)
at AnonymousObserver.Rx.AnonymousObserver.AnonymousObserver.completed (E:\localnet\birdknife\node_modules\vorpal\node_modules\inquirer\node_modules\rx-lite\rx.lite.js:1550:12)
at AnonymousObserver.Rx.internals.AbstractObserver.AbstractObserver.onCompleted (E:\localnet\birdknife\node_modules\vorpal\node_modules\inquirer\node_modules\rx-lite\rx.lite.js:1489:14)
at Subject.Rx.Subject.addProperties.onCompleted (E:\localnet\birdknife\node_modules\vorpal\node_modules\inquirer\node_modules\rx-lite\rx.lite.js:5871:19)
at Subject.tryCatcher (E:\localnet\birdknife\node_modules\vorpal\node_modules\inquirer\node_modules\rx-lite\rx.lite.js:63:31)
at AutoDetachObserverPrototype.completed (E:\localnet\birdknife\node_modules\vorpal\node_modules\inquirer\node_modules\rx-lite\rx.lite.js:5796:56)
at AutoDetachObserver.Rx.internals.AbstractObserver.AbstractObserver.onCompleted (E:\localnet\birdknife\node_modules\vorpal\node_modules\inquirer\node_modules\rx-lite\rx.lite.js:1489:14)
at AutoDetachObserver.tryCatcher (E:\localnet\birdknife\node_modules\vorpal\node_modules\inquirer\node_modules\rx-lite\rx.lite.js:63:31)
```