More improvements and refactoring to the helpers

This is the second round of refactoring changes building on the `simplified` branch. This contains the following changes:

* Make `{{intlGet}}` a generic helper to get anything on `options.data.intl`.
* Add support for `format()` methods passed to `{{intlMessage}}`.
* Only export `registerWith()` which allows helpers to reference Handlebars.
* Add `{{intlHTMLMessage}}` helper.