Allow extraSignupField "name".

The existing implementation used "name" as a special property in extraSignupFields, which meant that if an extraSignupField called "name" was added (which is used for the full name in accounts-admin-ui ) it had a default value of "name".

I've just renamed it to "field", while passing through some additional attributes, so I can have a field called "name". Example:

```json
                    "extraSignUpFields": [{
                            "field": "name",
                            "name":"This Will Be The Initial Value",
                            "label": "Full Name",
                            "placeholder": "John Doe",
                            "type": "text",
                            "required": true
                        }]
```

**Note that this may break some implementations.**