function config (options:RNFetchBlobConfig) {
  return { fetch : fetch.bind(options) }
}
Calling this method will inject configurations into followed `fetch` method.
@param  {RNFetchBlobConfig} options
        Fetch API configurations, contains the following options :
        @property {boolean} fileCache
                  When fileCache is `true`, response data will be saved in
                  storage with a random generated file name, rather than
                  a BASE64 encoded string.
        @property {string} appendExt
                  Set this property to change file extension of random-
                  generated file name.
        @property {string} path
                  If this property has a valid string format, resonse data
                  will be saved to specific file path. Default string format
                  is : `RNFetchBlob-file://path-to-file`
        @property {string} key
                  If this property is set, it will be converted to md5, to
                  check if a file with this name exists.
                  If it exists, the absolute path is returned (no network
                  activity takes place )
                  If it doesn't exist, the file is downloaded as usual
        @property {number} timeout
                  Request timeout in millionseconds, by default it's 30000ms.

@return {function} This method returns a `fetch` method instance.