#### Summary

Axios rejects error with Error type. Adding custom error type will help with distinguishing between axios and other errors. The only way now is to check 'request' field existence which is not always possible. Our internal custom error type has this field too.

#### Example:
``` js
const { axios, AxiosError } = require('axios');
...
try {
  ...
  const response = await axios.get(url);
  ...
} catch (error) {
  if(error instanceof AxiosError) {
    return processAxiosError(error);
  }
  throw error;
}
```

#### Context

- axios version: 0.18.0

Relates to: #24
index.d.ts
export interface AxiosTransformer {
  (data: any, headers?: any): any;
}
export interface AxiosAdapter {
  (config: AxiosRequestConfig): AxiosPromise<any>;
}
export interface AxiosBasicCredentials {
  username: string;
  password: string;
}
export interface AxiosProxyConfig {
  host: string;
  port: number;
  auth?: {
    username: string;
    password:string;
  };
  protocol?: string;
}
export type Method = 
  | 'get' 
  | 'delete' 
  | 'head' 
  | 'options' 
  | 'post' 
  | 'put' 
  | 'patch'
export type ResponseType = 
  | 'arraybuffer' 
  | 'blob' 
  | 'document' 
  | 'json' 
  | 'text' 
  | 'stream'
export interface AxiosRequestConfig {
  url?: string;
  method?: Method;
  baseURL?: string;
  transformRequest?: AxiosTransformer | AxiosTransformer[];
  transformResponse?: AxiosTransformer | AxiosTransformer[];
  headers?: any;
  params?: any;
  paramsSerializer?: (params: any) => string;
  data?: any;
  timeout?: number;
  withCredentials?: boolean;
  adapter?: AxiosAdapter;
  auth?: AxiosBasicCredentials;
  responseType?: ResponseType;
  xsrfCookieName?: string;
  xsrfHeaderName?: string;
  onUploadProgress?: (progressEvent: any) => void;
  onDownloadProgress?: (progressEvent: any) => void;
  maxContentLength?: number;
  validateStatus?: (status: number) => boolean;
  maxRedirects?: number;
  socketPath?: string | null;
  httpAgent?: any;
  httpsAgent?: any;
  proxy?: AxiosProxyConfig | false;
  cancelToken?: CancelToken;
}
export interface AxiosResponse<T = any>  {
  data: T;
  status: number;
  statusText: string;
  headers: any;
  config: AxiosRequestConfig;
  request?: any;
}
export interface AxiosError extends Error {
  config: AxiosRequestConfig;
  code?: string;
  request?: any;
  response?: AxiosResponse;
}
export interface AxiosPromise<T = any> extends Promise<AxiosResponse<T>> {
}
export interface CancelStatic {
  new (message?: string): Cancel;
}
export interface Cancel {
  message: string;
}
export interface Canceler {
  (message?: string): void;
}
