Add configurations/hooks for fields, include, and links.

[beauby]

Closes #50.

[smaximov]

I'm afraid this won't work out of the box.

params[:fields] would return AC::Parameters, which is not compatible with Hash in Rails 5. So we can use something like params[:fields]&.to_unsafe_hash instead. But this won't work either because fields for every resource specified still need to be parsed (for instance, a request to /api/posts?fields[posts]=title,summary&fields[comments]=content,rating will result in a :fields hash { "posts" => "title,summary", "comments" => "content,rating" }, which will cause an error if passed to the renderer).

So I believe it will be safe to disable parsing/processing of JSONAPI fields by default (set it to nil or an empty hash) and provide a parser, just like you did with jsonapi_include.

[beauby]

Updated the PR according to your feedback. At the moment I simply provide an example in the initializer but I agree it might make sense in the future to:

1. provide Hash[fields_param.map { |k, v| [k.to_sym, v.split(',').map!(&:to_sym)] }] as a method somewhere,
2. expose some mechanism to parse the fields param by default without necessarily using it (i.e. make it into a hash and turn it back to an AC::Parameters instance that the user can directly validate).

[smaximov]

Cool, I like it. I can test this PR (the fields part) in an hour on my website.

[beauby]

Awesome, thanks!

[smaximov]

Tested jsonapi_fields configuration, works without any issue.

[smaximov]

Tested jsonapi_fields configuration, works without any issue.