Add comma-dangle eslint rule for trailing commas

[ntwb]

Via WordPress/gutenberg#612 and WordPress/gutenberg#741

Let's add a lint rule for trailing commas:

const obj = {
    aubergine: 1,
    tomato: 2,
};

const arr = [
    'salmon',
    'shrimp',
];

There are several advantages to this. From the WordPress coding standards for PHP:

Note the comma after the last array item: this is recommended because it makes it easier to change the order of the array, and makes for cleaner diffs when new items are added.
In the past, this has been avoided in JavaScript due to lack of support in older versions of IE (this didn't work correctly until IE9). However, we don't have to care about this because we have a build process to handle it for us and because WordPress is dropping support for IE 8-10.

Rule: http://eslint.org/docs/rules/comma-dangle

Rule configuration suggestion: 'comma-dangle': ['error', 'always-multiline'],

WordPress JavaScript Coding Standards docs that will need to be updated:
• See https://make.wordpress.org/core/handbook/best-practices/coding-standards/javascript/#objects
• See https://make.wordpress.org/core/handbook/best-practices/coding-standards/javascript/#arrays-and-function-calls
• See https://make.wordpress.org/core/handbook/best-practices/coding-standards/javascript/#examples-of-good-spacing
• See https://make.wordpress.org/core/handbook/best-practices/coding-standards/javascript/#underscore-js-collection-functions

Previously: e911437