test() does not support -w and -x options

These are sometimes useful and are not supported as of latest shelljs (v0.7.8):

- -w file exists and write permission is granted
- -x file exists and execute permission is granted

Usage:
```javascript
require('shelljs').test('-x', '/bin/bash')
// => test: option not recognized: x

```

It can be implemented using `fs.accessSync`. Would you be interested in a PR?