Unexpected Exit on Grep.

Initially I attempted a "grep" to search for a string inside a file using the -irl flags and found that the -L (lowercase) was not available but then tried the -n flag instead

Command executed

```
grep -irn "test" *
```

cash exits to windows prompt.

verification of error.

```
root:C/Users/Public/Documents$ root:C/Users/Public/Documents$ mkdir test1, test2, test3

root:C/Users/Public/Documents$ ls
desktop.ini My Music My Pictures My Videos test1, test2, test3

root:C/Users/Public/Documents$ grep -irn "test" *
readline.js:925
throw err;
^

SyntaxError: Invalid regular expression: /(*)/: Nothing to repeat
at RegExp (native)
at new RegExp (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\node_modules\babel-polyfill\node_mo
dules\core-js\modules\es6.regexp.constructor.js:26:11)
at CommandInstance.exec (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal-grep\dist\grep.js:23:19)
at CommandInstance. (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal-grep\dist\grep.js:
97:15)
at EventEmitter.session.execCommandSet (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\sessi
on.js:476:20)
at EventEmitter.vorpal._exec (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\vorpal.js:970:1
8)
at EventEmitter.vorpal._execQueueItem (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\vorpal
.js:761:17)
at EventEmitter.vorpal._queueHandler (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\vorpal.
js:745:10)
at EventEmitter.vorpal.exec (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\vorpal.js:692:10
)
at C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\vorpal.js:635:10
readline.js:925
throw err;
^

SyntaxError: Invalid regular expression: /(*)/: Nothing to repeat
at RegExp (native)
at new RegExp (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\node_modules\babel-polyfill\node_mo
dules\core-js\modules\es6.regexp.constructor.js:26:11)
at CommandInstance.exec (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal-grep\dist\grep.js:23:19)
at CommandInstance. (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal-grep\dist\grep.js:
97:15)
at EventEmitter.session.execCommandSet (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\sessi
on.js:476:20)
at EventEmitter.vorpal._exec (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\vorpal.js:970:1
8)
at EventEmitter.vorpal._execQueueItem (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\vorpal
.js:761:17)
at EventEmitter.vorpal._queueHandler (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\vorpal.
js:745:10)
at EventEmitter.vorpal.exec (C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\vorpal.js:692:10
)
at C:\Users\$user\AppData\Roaming\npm\node_modules\cash-global\node_modules\cash\node_modules\vorpal\dist\vorpal.js:635:10
```

I don't know if this output will help and I'm not sure if there is an issue with an thirdparty .js, however I did test this exit with the following commands just to verify as well.. same result of exiting.

```
grep -irn "test*" *
grep -irn 'test*' *
grep -irn 'test' *
```