babel-eslint support

Currently, `vue-eslint-parser` is using the default parser (espree).
It needs the capability to use custom parsers to parse `<script>` element.

For example:

```json
{
    "parser": "vue-eslint-parser",
    "parserOptions": {
        "vue-eslint-parser": {
            "parser": "babel-eslint"
        },
        "sourceType": "module",
        "ecmaVersion": 2017
    }
}
```
