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?

Start by reading hyper-spec's client_options layout handling and the existing javascript option, then compare that path with the webpack setup shown in the issue. Determine how the standard webpack entry should be represented and verify that generated layouts include the needed hyperstack asset without the application-layout workaround.

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