#### Summary

It would be nice to export a typeguard from Axios' index.d.ts.
This way, consumers trying to do error handling can check in a type-safe way if the error is an AxiosError, or some other kind of error.

It's easy enough to do this as a consumer, but it would be nicer if this was included in the actual Axios library.

test/specs/instance.spec.js
it('should make an http request with url instead of baseURL', function (done) {
    var instance = axios.create({
      url: 'https://api.example.com'
    });
    instance('/foo');
    getAjaxRequest().then(function (request) {
      expect(request.url).toBe('/foo');
      done();
    });
  });
  it('should make an http request', function (done) {
    var instance = axios.create();
    instance.get('/foo');
    getAjaxRequest().then(function (request) {
      expect(request.url).toBe('/foo');
      done();
    });
  });
  it('should use instance options', function (done) {
    var instance = axios.create({ timeout: 1000 });
    instance.get('/foo');
    getAjaxRequest().then(function (request) {
      expect(request.timeout).toBe(1000);
      done();
    });
  });
  it('should have defaults.headers', function () {
    var instance = axios.create({
      baseURL: 'https://api.example.com'
    });