Include track numbers

[amiel]

This should resolve both #17 and resolve #38.

• by default this comes from the track number in metadata
• for playlists this comes from the order of the playlist

[flxo]

Love this! Anything I can do to get this merged?

[MicaEcho]

it can cause unexpected behaviour, duplicating and redownloading files that are already available locally

1. if the local file name match artist - song (with no track number), it should rename the file with the track number instead redownload
2. if the local file name have a wrong track number (as when playlist order change), it should also update file names instead redownload