add more setup/invocation options: onError (function), captureResults (integer)

[saadtazi]

See Proposal in #1 . Interface has changed a little bit: available options:

{
  // new options...
  // the last 10 results will be stored and will be available in `onError`... You can use `Infinity` if needed
  captureResults: 10,
  // error handler. errorType is `retries` or `duration`
  onError({ errorType, reject, nbAttempts, startedAt, capturedResults }, options) {
    //... return error(s)...
    const err = new Error('....');
    // attach properties to error if you want, for example...
    err.capturedResults = capturedResults;
    reject(err);
  }
  // also available...
  // default Promise A+ library (defaults: native Promise)
  Promise: Bluebird,
  // default number of retries before rejecting - default: Infinity
  retries: 10,
  // time to wait between 2 calls - default: 0
  wait: 5,
  // max number of milliseconds before rejecting - default: Infinity
  duration: 25
}

Those options can be used when invoking setup() but also until(func, testfunc, options).

[coveralls]

Coverage increased (+2.8%) to 100.0% when pulling 819d4f0 on feature/onError-handler-and-captured-results into 0f89442 on master.