Aliased values treated as strings

This works:

```javascript
var x = parse([ '-s',  '000123' ], { 
    string: 's',
    alias: { s: 'str' }
});
```
`x.s` and `x.str` are both the string `'000123'`.

This fails:

```javascript
var x = parse([ '--str',  '000123' ], { 
    string: 's',
    alias: { s: 'str' }
});
```

`x.s` and `x.str` are both the number `123` instead of the string `'000123'`.

- - -

The pull request adds the aliased values into`flag.strings` if they are in the `opts.string` array.