Adding CSS classes to thead, tfoot and tbody

@fran-worley https://github.com/offirgolan/ember-light-table/issues/99#event-734291228:

> Sorry to bump an old issue but I believe this only partially resolves issues marrying up Bootstraps css styles with ember-light-table.
>
>We need to be able to apply styles directly to the 'thead' too ('.thead-default' is just one style example from BS v4). Could the api be harmonized to allow you to add classes directly to table elements (table, thead, tbody etc.).

As a user, I would expect that setting `class` on `{{t.head}}`, `{{t.body}}` and `{{t.foot}}` set these classes on the rendered ``, `` and `` elements.

I'm not sure, if we can use `class` as the attribute name here, since it's a magic `Component` keyword that automatically sets the `class` HTML attribute for the rendered component element.

The issue names the contextual components t.head, t.body, and t.foot but no files or tests. Start by locating those component entry points and inspect how their class attributes are handled for the rendered thead, tbody, and tfoot elements. Done means classes supplied on each component appear on the corresponding table section without breaking existing usage.

索引モデルが issue の本文から書いたものです。