Add tests for HTMLBars attribute hook/helper.

* Add tests to confirm we are using `setAttribute` for some level of
  escaping (equivalent to what we have today with `{{bind-attr}}`).
* Updated to ensure that rerenders happen in the `render` queue.
* Added dirty tracking to not needlessly update attributes.
* Move `attribute` and `concat` to separate `helpers/` files.
* Changed `lookupHelper` to be a default export.