Add path.common(path1/*, ...pathn*/);

```javascript
var path = require('path');

path.common('/lorem/ipsum/foo/bar', '/lorem/ipsum/raz/dwa', '/lorem/elo/foo/bar'); //  => '/lorem'

path.common('/foo/bar', '/raz/dwa'); // => '/'

path.common('C:\\\\foo\\bar', 'D:\\\\foo\\bar'); // => null
```

I have use case for it in few of my modules (using counterparts for now), and I wonder why it's not part of `path` API.

I'll be happy to provide a pull request, but before that I'd like to be sure that you're open for such add on.