Instance does not have global interceptors

#### Summary

I searched through all the issues and double checked the ones referenced in https://github.com/mzabriskie/axios/issues/812 but my problem seems to be different.

My problem is that creating an instance does not copy over or use the interceptors added to the global Axios.

import Axios from "axios";

Axios.interceptors.request.use((config) => {
  console.log("Global Interceptor")
  return config;
});

const axiosInstance = () => {
  let instance = Axios.create();
  console.log(Axios.interceptors.request); // Has 1
  console.log(instance.interceptors.request); // Has 0
  return instance;
}

axiosInstance().post("/test", { testing: "data" }); // Doesn't output "Global Interceptor"
Axios.post("/test", { testing: "data" }); // Outputs "Global Interceptor"

#### Context

• Axios version: 0.16.2
• Environment: Webpack 2.6.1, Chrome Canary 61.0.3147.0, Mac OS X

[bierik]

@rubennorte I would suggest not adding the feature of global interceptors. What about extending an existing axios instance. This way the developer exactly knows what interceptors and options are inherited or extended:

import axios from 'axios';

export const api = axios.create({
  baseURL: process.env.API_ROOT,
});

api.interceptors.request.use((request) => {
  request.headers.Authorization = `Bearer mytoken`;
  return request;
});

// Here the pdfInstance extends the api instance so the pdfInstance has also access to the request interceptor but is also able to override the baseURL and adding additional header such as Accept
export const pdfInstance = api.extend({
  baseURL: '/file',
  responseType: 'arraybuffer',
  headers: {
    Accept: 'application/pdf',
  },
});

[EvanK]

What about a convenience function to pull in interceptors from an existing instance:

// from the default instance
let api = axios.create({baseUrl:"http://internal.mycompany.com"});
api.interceptors.assimilate(axios);

// or from another created instance
let thirdParty = axios.create({baseUrl:"https://partner.example.com"});
thirdParty.interceptors.assimilate(api);

On top of that, you could explicitly pull interceptors from the default instance with a config option:

let api = axios.create({defaultInterceptors:true});