Support for a nicer structure for ID maps

[djmattyg007]

This is a proof of concept for #11 that allows use of a JSON ID map that looks like this:

[
  {
    "primary_id": "1",
    "alternate_ids": ["2", "5", "11"]
  },
  {
    "primary_id": "4",
    "alternate_ids": ["17", "82"]
  }
]

Because of how the code is structured, there's nothing stopping you from using an ID map that looks like this:

[
  {
    "name": "Friend 1",
    "primary_id": "1",
    "alternate_ids": ["2", "5", "11"]
  },
  {
    "name": "Friend 2",
    "primary_id": "4",
    "alternate_ids": ["17", "82"]
  }
]

If you're happy with this implementation, I'll start updating the docs, and add some tests too.

[djmattyg007]

I rebased my commits against the lastest changes on the master branch, squashed it all down into one commit, and pushed again. Hope it all looks good :)

[djmattyg007]

I've fixed the name of the test.

[Thor77]

Thanks for your contribution 🎉