Allow setting input and output (fixes issue with busy process.stdin)

Very useful with [ttys](https://github.com/TooTallNate/ttys) to allow querying user even if stdin is already used.

I use this modification in a script that should prompt user from a git hook, this is the only way to make it work properly.

```js
var ttys = require("ttys");

inquirer.prompt([
  {"type": "input",     "message": "Github username", "name": "user"},
  {"type": "password",  "message": "Github password", "name": "password"}
], function (answers) {
  // …
}, {
  input: ttys.stdin,
  output: ttys.stdout
})
```
Make this an `if` statement.