Allow access to express-style path parameters in mocked endpoints

[ryanscottaudio]

When adding a mock for an express-style path (e.g. express:/example/:foo/:bar, there is no built-in way to access the parsed values for foo and bar:

const mockedEndpoint = (url, opts) => {
  // we should be able to use { foo: <whatever>, bar: <whatever> } here without additional parsing
};

fetchMock.get('express:/example/:foo/:bar', mockedEndpoint);

fetch('/example/foovalue/barvalue'); // we would have access to { foo: 'foovalue', bar: 'barvalue' } in mockedEndpoint 

We're giving fetchMock.get enough information (the matching express-style pattern and the URL itself) that it should be able to parse out what foo and bar are and pass them to the callback (potentially as a third argument?).

This issue was called out in #300 but either wasn't addressed at all or was addressed but in a too-opaque-for-me-to-understand way.