Add cancelToken

[stereobooster]

Example code (facebook/react#8883 (comment)):

  fetchRepos() {
    // cancel the previous request
    if (this.source) {
      this.source.cancel('Operation canceled due to new request.')
    }

    // save the new request for cancellation
    this.source = axios.CancelToken.source();

    this.setState({networkState: 'loading' })

    let url = `https://api.github.com/users/${this.props.username}/repos`
    axios.get(url, { cancelToken: this.source.token })
      .then(response => this.setState({ repos: response.data, networkState: 'loaded' } ))
      .catch(error => {
        if (axios.isCancel(error)) {
          this.setState({ networkState: 'canceled' })
        } else {
          this.setState({ networkState: 'errored' })
        }
     });
  }

[miguelcaravantes]

can we reopen this? there is not cancelToken parameter yet