classNames, cellClassNames cannot be arrays

https://github.com/offirgolan/ember-light-table/blob/v1.8.6/addon/classes/Column.js#L259-L265
```js
/**
* Class names to be applied to all cells of this column
*
* @property cellClassNames
* @type {String | Array}
*/
cellClassNames: null,
```

https://github.com/offirgolan/ember-light-table/blob/v1.8.6/addon/components/cells/base.js#L15-L25
```js
/**
* @module Cell Types
* @class Base Cell
*/

const Cell = Component.extend({
layout,
tagName: 'td',
classNames: ['lt-cell'],
attributeBindings: ['style'],
classNameBindings: ['align', 'isSorted', 'column.cellClassNames'],
// ..
});
```

The same goes for `classNames`.

When passed an array of class names, the array is stringified (concatenated with `,`).

I'll submit a PR when I can get around to it. Merely putting this here for tracking. :slightly_smiling_face:
Otherwise this would be a great PR for a first-time contributor! :smile: