Add docs, download multiple files, support Python 3, fix ffmpeg command

• Added README.md so that a less technically inclined person could also use the script.
• Added support for downloading multiple files by specifying a single TSV file with (filename, URL) pairs as input.
• Fixed some statements (prints and lambdas) to be able to run on Python 3. As far as I'm aware, these changes should not break Python 2 compatibility either.
• Removed the -c:v copy -c:a aac flags from ffmpeg command line. The issue I found is that for some videos, when you try to use copy for video, it unfortunately results in a broken output file which cannot be played past a certain point. I suspect that this is because M4V streams cannot be always concatenated smoothly. Maybe the issue happens when the split happens between key frames or something. Anyway, without those options ffmpeg will just recode both audio and video in a sensible way. Even though this will take significantly longer (though still not longer than the video's runtime, even on low performance machines), this will guarantee the output files will not be broken.