Add examples for common uses of the API.

[Robin]

• Parse metadata of file.
  • flac.ParseFile
• Open a file for audio streaming without parsing metadata.
  • flac.Open
• Parse the audio samples within audio frames:
  • flac.Stream.ParseNext
• Seek to a specific audio frame without parsing any audio samples:
  • flac.Stream.Next

ref: https://blog.golang.org/examples