Teach lyrics plugin to look for dual song lyrics separately.

  % beet -v lyrics -f title::' / ' 'Pink Floyd'
  Attempting to fetch http://lyrics.wikia.com/Pink_Floyd:Speak_to_Me_/_Breathe from LyricsWiki
  Attempting to fetch http://lyrics.wikia.com/Pink_Floyd:Speak_to_Me from LyricsWiki
  got lyrics from backend: fetch_lyricswiki
  Attempting to fetch http://lyrics.wikia.com/Pink_Floyd:Breathe from LyricsWiki
  got lyrics from backend: fetch_lyricswiki
  Found lyrics for Speak to Me and Breathe after searching separately