hyper-spec not working with webpack setup

looks like if webpack is in use then the standard layout looks like:

```erb
<%= javascript_pack_tag 'hyperstack' %>
<%= javascript_include_tag 'application', 'data-turbolinks-track': 'reload' %>
```

but hyper-spec has not been trained to do this, and only includes the application tag by default.

Workaround for now is to add a `client_options layout: 'application'` in a before(:all) block in your spec helper. This will force hyper-spec to just use the application layout instead of building its own.

Solution is probably to add a webpack option that works like the hyper-spec javascript option, and then to default this value to `hyperstack`...

@barriehadfield can you weigh in... would we ever have a whole list of webpacks, or is it always going to be `hyperstack` for a standard setup?