Add command line options for skip and successful file

[linusg]

Closes #296.

Adding two command line flags: -sk/--skip and -w/--write-successful. Both have the same format, so using the same file with both flags is possible:

spotdl -l playlist.txt -w skip.txt -sk skip.txt

I.e. successfully downloaded tracks will land in skip.txt and thus being skipped directly when creating and downloading the track file again.

[linusg]

Also I fixed a bug where the tracks file would contain multiple songs in one line:

Since we use '\n'.join(...) for (over)writing the file, there will be no newline at the end.
(Verified by returning right after this, but the theory is simple anyway.)

https://github.com/ritiek/spotify-downloader/blob/e076d11a19672b1b08119dce846358747c203df7/spotdl/spotdl.py#L74

When we then append raw_song + '\n', it will be on the same line as the last song!

https://github.com/ritiek/spotify-downloader/blob/e076d11a19672b1b08119dce846358747c203df7/spotdl/spotdl.py#L97

[ritiek]

This line is really long. Best we break it into lines like:

download_list(tracks_file=const.args.list,
              skip_file=const.args.skip,
              write_successful_file=const.args.write_successful)

[ritiek]

I left a minor comment but looks good to me overall. We could merge this once it has been addressed.

[linusg]

You're right, that was quick copy and paste 😉
I'd love to introduce the usage of a style guide/autoformatter (I personally love black), but that's another issue.

[ritiek]

Let's merge this. Thanks @linusg!