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`.
`#delete` takes a default, like `#fetch`
```
def render_ember_handlebars?(options = {})
   options.delete("ember") { ember }
end
```