Exempt variables prefixed with underscore from no-unused-vars rule (#640)

* 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>;
    }

config/eslint.js

@@ -131,7 +131,11 @@ module.exports = {
     'no-unreachable': 'warn',
     'no-unused-expressions': 'warn',
     'no-unused-labels': 'warn',
-    'no-unused-vars': ['warn', { vars: 'local', args: 'none' }],
+    'no-unused-vars': ['warn', {
+      vars: 'local',
+      varsIgnorePattern: '^_',
+      args: 'none'
+    }],
     'no-use-before-define': ['warn', 'nofunc'],
     'no-useless-computed-key': 'warn',
     'no-useless-concat': 'warn',