Add option to define which element attributes to use for key creation

I have a use case where the `name` of my form elements change, but the `id` stays the same. By leaving out the name attribute jquery-phoenix works. This PR allows me to change the way `storageKey` is created.

e.x.
```javascript
$inputs.phoenix({
  keyAttributes: ['tagName', 'id']
});
```
Any objections?

Thanks for the great jquery plugin btw!