Allow "if: Proc..." on link

[listrophy]

Just like attribute, this PR allows the dev to specify links as follows:

link(:download, if: ->(obj, params) { params && params[:signed_url] }) do |obj, params|
  params[:signed_url]
end

[jopotts]

Looks like an improvement. You're welcome to add the PR here too: jsonapi-serializer/jsonapi-serializer#1