Add support for arbitrary external types

[qwtel]

Not quite what was discussed in #69, but maybe useful anyway.

This PR adds support for the externalTypes options keys, which accepts values like

const transformer = new Transformer({
  externalTypes: {
    "@platformparity/streams": ["ReadableStream"]
  }
});

where the key is an import path, and the value is an array of named exports.

This was built with ReadableStream in mind, which is used by the Fetch API, but doesn't have a WebIDL definition itself. However, there is a reference implementation which can be used and type-checked with this change.

It can also be used to use node 10's built-in URL and URLSearchParams types without having to modify the webidl-conversions package, e.g.

  externalTypes: {
    "url": ["URL", "URLSearchParams"]
  }