Version 0.7.1

• Add ffmpeg decoding support; this is optional and currently requires building from source
• Fix a math overflow on 32-bit for MSSSIM
• Publish a new av_metrics_decoder crate, to use the y4m and ffmpeg decoders independently.
  These are re-exported through av-metrics, so the av-metrics interface is unchanged.
• Add progress indicator
• Remove internal unwraps (enables cleaner exiting, especially when used as a crate)
• Improve error messages