Helps to build interpters of HAL documents.

Given a document like

{
  "name": "Jane Doe",
  "mailingAddress": "123 Main St",
  "_links": {
    "app:department": { "href": "http://example.com/dept/42" }
  }
}

We can define an interpreter for it.

defmodule PersonInterpreter do
  use ExHal.Interpreter

  defextract :name
  defextract :address, from: "mailingAddress"
  defextractlink :department_url, rel: "app:department"
end

We can use this interpreter to to extract the pertinent parts of the document into a map.

iex> PersonInterpreter.to_params(doc)
%{name: "Jane Doe",
  address: "123 Main St",
  department_url: "http://example.com/dept/42"}