Add proxy option which can override the http(s) proxy which might be configured in environment variables.

• opts.proxy: undefined, false, or object as defined in axios. Default is undefined. If not set (undefined) axios detects proxy from env vars http_proxy and https_proxy. https://github.com/axios/axios#config-defaults

  // example proxy object
  {
    host: '127.0.0.1',
    port: 9000,
    auth: {
      username: 'mikeymike',
      password: 'rapunz3l'
    }
  }