Implement URI fragment link relations and fix dereferencing

[danielgtaylor]

This fixes the bug introduced with the 1.0.1 release, as well as adding in the new link relation uri-fragment which contains the unique URL-safe ID as its href value. E.g.

let element = {
  element: 'string',
  meta: {
    id: 'My Special String'
  },
  content: 'Hello!'
};

abagnale.forge(element);

console.log(element.meta.id);
// => My Special String
console.log(element.meta.links[0].href);
// => my-special-string

Obviously you shouldn't assume links[0] but you get the idea! 😄 Object members get separated by / instead of . for the URI fragment, so you get pieces like my-object/member-name. Both separator types are configurable and the README is updated to reflect that.

cc @Baggz