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>;
    }

config/eslint.js

@@ -133,6 +133,7 @@ module.exports = {
     'no-unused-labels': 'warn',
     'no-unused-vars': ['warn', {
       vars: 'local',
+      varsIgnorePattern: '^_',
       args: 'none'
     }],
     'no-use-before-define': ['warn', 'nofunc'],