Add ability to select multiple attribute keys

Hi guys,

I find useful to be able to pass multiple attributes to the `.select` method.

Example:
```javascript
this.defaultAttrs({
  menuTitleSelector: '.menu-title',
  menuItemSelector:  '.menu-item'
});

this.selectMenu = function(e) {
  return this.select('menuTitleSelector', 'menuItemSelector');
};
```

What do you think?

Thanks!