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

Version 0.4.0

• Breaking Change: Use Frame, Pixel, etc. types from the v_frame crate,
  instead of rolling our own. This should improve interoperability
  with other crates.
• Speed up y4m decoding.
• Minor internal changes and dependency updates.