Add asyncData() support for the subcomponents

[pkarw]

Somehow related to:
nuxt/nuxt#32 (comment)

https://nuxtjs.org/faq/async-data-components/

Why loading the data into core/pages is not optimal solution? Because it breaks the SRP principle - when You like to add your let's say ProductDetails component to other page You need to somehow duplicate the data fetching logic. Of course, we can move the more-sophisticated data fetching logic solely to Vuex actions to simplify the Components logic as a first step

We need to modify:
https://github.com/DivanteLtd/vue-storefront/blob/9be57858de241ab7f0d39b999cedae137fb0b40b/core/client-entry.js#L27

To support not only mixins but also components. This will allow us to refactor the core/pages to more granular components like: ProductDetails etc.

The sub-components could be more self-sufficient then and take the responsibility for it's own state management - but not only!

Open questions:

• how about properties support (the asyncData is static - doesn't have access to this)?

Roadmap:

• add asyncData support for child components,
• simplyify the data fetching logic for Product and Category pages - moving the fetchData logick to Vuex action
• create more granular components that can load product details, list of products using only properties - without router access
• try to refactor core/pages/Product and core/pages/Category using these components