[Feature Request] Inject the form model into the custome validator function

### Existing Component
是

### Component Name
el-form

### Description
Without model in validator function, I have to use this.xxx or $refs to access the model itself. It's inconvenient when I use config-like style to write a form validation.Especially when the config and the form component codes are in different repos which communicate with server's json response.

e.g

in repo1 there's component's code:

```html

```

in repo2 there's form items' config:
```javascript
{
label: 'xxx',
rule: [
validator(rule, value, callback) {
// how to access another field's value? Can I still use "this" keyword?
}
]
}
```
I cannot access other fields in the scene like this.
Thank you.