allow Vorpal to run arbitrary bash commands and pipe back to shell

My Vorpal options are like so:

```
Commands:

help [command...] Provides help for a given command.
exit Exits application.
pwd echo present working directory
run [file] run a single test script
find [options] find test files to run
```

I'd like to be able to type a command - if it's not a recognized option - to send it to bash and have bash interpret it.

One thing I could do is create a new option

` bash [command...] `

and then I could send all the arguments to bash that way..

however that requires users to type in something like:

`> bash "ls -a"`

or

`> bash "ps aux | grep node"`

like I said, it would be cool to be able to give Vorpal the option of sending unrecognized commands to bash and then piping the results back to vorpal terminal.