Add Track Tagging with id3

[stephen-huan]

Adding the TRCK header (Ffmpeg metadata tag's "track") parameter would be helpful to organize the resulting playlist, especially for playlists with a lot of songs.

For example, the TRCK tag is what determines the order in applications like cmus:

If I'm reading the code right, it should be as simple as this:
Assuming the songs are downloaded in order, then adding a number variable should work.
(file spotify_dl/spotify.py, lines 77-90)

    for number, item in enumerate(info):
        log.debug('Songs to download: %s', item)
        url_, track_, artist_ = item
        download_archive = download_directory + 'downloaded_songs.txt'
        outtmpl = download_directory + '%(title)s.%(ext)s'
        ydl_opts = {
            'format': format_string,
            'download_archive': download_archive,
            'outtmpl': outtmpl,
            'noplaylist': True,
            'postprocessor_args': ['-metadata', 'title=' + str(track_),
                                   '-metadata', 'artist=' + str(artist_),
                                   '-metadata', 'track=' + str(number + 1)]
        }

A bit of basic testing seems to work.

[SathyaBhat]

@stephen-huan my deepest apologies for missing this. I'll merge this and publish with the next release