feat: allow overriding components when extending

[lougreenwood]

This PR allows end users of the add-on to override the components which are used to assemble a table.

Our use case for this is a situation where we have our own in-house sc-table component which extends ember-light-table. However, we need to apply additional styling to the sub-components used by ember-light-table using ember-css-modules.

For example, by overriding the components, we can create our own private-lt-head component, and override the headComponent value in light-table.js:

sc-light-table.js (our private table component which overrides ember-light-table

import LightTable from 'ember-light-table/components/light-table'
import styles from 'styles/components/sc-light-table'
import { classNames } from '@ember-decorators/component'

@classNames(styles['sc-light-table'])
export default class ScLightTableComponent extends LightTable {
  /**
   * @property headComponent
   * @type {String}
   * @private
   */
  headComponent = 'sc-light-table/head'

  /**
   * @property bodyComponent
   * @type {String}
   * @private
   */
  bodyComponent = 'sc-light-table/body'

  /**
   * @property footComponent
   * @type {String}
   * @private
   */
  footComponent = 'sc-light-table/foot'
}

Usage is then as simple as: <ScTable ... /> and we get ember-light-table with custom sub-components.