+ progress_hooks: A list of functions that get called on download
+ progress, with a dictionary with the entries
+ * filename: The final filename
+ * status: One of "downloading" and "finished"
+
+ The dict may also have some of the following entries:
+
+ * downloaded_bytes: Bytes on disk
+ * total_bytes: Size of the whole file, None if unknown
+ * tmpfilename: The filename we're currently writing to
+ * eta: The estimated time in seconds, None if unknown
+ * speed: The download speed in bytes/second, None if
+ unknown
+
+ Progress hooks are guaranteed to be called at least once
+ (with status "finished") if the download is successful.
+