Get a download ID.
Returns a download id and token, or null if a callback is specified.
An object containing table download options with the following possible values:
- table: The feature collection to download.
- format: The download format, CSV, JSON, KML, KMZ or TF_RECORD.
- selectors: List of strings of selectors that can be used to determine which attributes will be downloaded.
- filename: The name of the file that will be downloaded.
An optional callback. If not supplied, the call is made synchronously.