add pause-metadata option when creating a new bt task

[WeidiDeng]

When creating a bt task, it's possible user wants to select the files of the torrent to download. Instead of downloading and pausing to select the files, pause-metadata allows the user to make the selection on the paused task and start it.

[mayswind]

AriaNg already has the feature you mentioned. When you click "Download Later" during the new task page, it will create a paused task, allowing you to select the files you want to download first, and then start the download.

[WeidiDeng]

For torrent, and magnet links, there are differences.

Download Later will create the bt task as paused, there are no files to select because nothing is known about the contents of the torrents.

pause-metadata will download the torrents, then a new task will be created that allows to select the contents of the torrent.