Add option to customize the returned Content JSON data

[melvinsembrano]

Add option to customize the returned Content JSON data

This PR will add an option for dev to customize the content JSON data.

This will enable JS-Client (e.g. React, VueJS apps) to use the page fragments and custom display it on client apps.

[GBH]

Thanks. Merged with some changes:

• renamed it to page_to_json_options
• page has accessor for content so we can assign to it.
• default is methods: [:content], except: [:content_cache]. That way you can exclude :content. Before it was injected into the hash whenever you want that or not.