* Split no-unused-vars ESLint config to multiple lines

* Exempt variables prefixed with underscore from no-unused-vars rule

This is useful when e.g. using object spread operator to remove only a
certain field from the object.

For example, this can be used to ignore a property from React
component's `this.props`:

    render() {
        const { someAttribute: _unused, ...rest } = this.props;
        return <pre>{ JSON.stringify(rest) }</pre>;
    }