Major rewrite; Epub Chapter Support and Mp3 Output

• Pass in --mp3 to get mp3 output. (Useful for reducing size of the audio file compared to wav)
  • requires ffmpeg in your PATH
• Pass in --chapters when converting an epub file to get a mp3 file with chapter metadata
  • Implemented an epub parser internally to accomplish this.
  • Chapter metadata is encoded as ID3 tags; make sure your mp3 player supports this; many do. BookPlayer on iOS or mpv on Linux both work well.
  • Pass in --threads to control how many go routines are spawned while converting. However, you shouldn't need to touch this unless it is too high from some reason.
• Logging should be improved and more useful. Use --verbose for debug level logging.
• Internals of the codebase are much cleaner
  • More testing and better package isolation; more tests for Chinese