Default rails tag helpers to handlebars inside em_view

Now you do not need to pass options through to get rails to use handlebars.

This will be the default whenever there are tags inside of the `em_view` block.

You can still override the default functionality by passing `ember: true` or `ember: false`.
Can we extract the mode detection into a method? Maybe something like

```ruby
def render_ember_handlebars?(options = {})
   options.delete("ember") || ember
end