update audio quality argument handling

the value of -aq could be 0, but it's not working when you set .withAudioQuality(0) in original code
Merge branch 'master' of https://github.com/kejjang/node-fluent-ffmpeg

# By Kej
# Via Kej
* 'master' of https://github.com/kejjang/node-fluent-ffmpeg:
  update audio quality argument handling
updated coverage.html