Dirname array

Hi guys,

I had the need to scan more than one directory at once. Currently the "dirname" option only allows strings paths.

This PR add support to strings and arrays in dirname, It means that you can put more than one path to scan for include files. For example, you can do this:

```javascript
var multipleDirsControllers = requireAll({
  dirname: [
    __dirname + '/controllers',
    __dirname + '/other-controllers'
  ],
  filter: /(.+Controller)\.js$/,
  flattenDirectories: true
});
```
Instead of make two calls, merging results or any thing else.

I hope you find it useful