Version 0.3.0

• Breaking Change: Remove the use_simd flag from the public API.
  This was intended only for development purposes,
  and isn't generally useful to end users.
  If you really want to disable SIMD,
  you can set the environment variable AV_METRICS_DISABLE_SIMD to any value.
• Breaking Change: PSNR and APSNR have been split into separate result sets.
  This only impacts users of the API.
  The CLI output is identical.
• New Feature: --metric flag allows the CLI tool to output only one metric at a time,
  instead of all metrics.
• New Feature: --json flag allows the CLI tool to output the results as JSON.
  This is primarily useful if you want to use the output in some other script
  or as data on a web page.
• 25% speed improvement in CIEDE2000 with AVX2.