Allow expectedChallenge to return a Promise

[jordanbtucker]

This PR allows verifyAuthenticationResponse and verifyRegistrationResponse to accept expectedChallenge properties of their options parameter to be Promise returning functions. This allows code like this:

async function customAsyncVerification(challenge: string): Promise<boolean> {
   ...
}

const isVerified = await verifyAuthenticationResponse({
  ...
  expectedChallenge: customAsyncVerification,
  ...
});

Three tests per verification function have been added:

1. Checks for Promises that resolve with true.
2. Checks for Promises that resolve with false.
3. Checks for Promises that reject.

Fixes #431