v0.6.0

• Support decimal crf values in sample-encode, encode subcommands (note svt-av1 only supports integer crf).
• Add crf-search, auto-encode arg --crf-increment. Previously this would always be 1.
  Defaults to 1. -e libx264, libx265 & libvpx-vp9 default to 0.1.
• Add crf-search, auto-encode arg --thorough which more exhaustively searches to find
  a crf value close to the specified min-vmaf.
• Cache sample-encode results in $CACHE_DIR/ab-av1 directory. This allows repeated same crf sample encoding
  to be avoided when running sample-encode, crf-search & auto-encode. E.g. repeating a crf-search with
  a different min-vmaf.
  
  Caching is enabled by default. Can be disabled with --cache false or setting env var AB_AV1_CACHE=false.
• Use mkv containers for all lossless samples. Previously mp4 samples were used for mp4 inputs, however in all test cases
  mkv 20s samples were better quality. This change improves accuracy for all mp4 input files.
• Default --max-crf to 46 for libx264 & libx265 encoders.
• Encode webm outputs with the "cues" seek index at the front to optimise stream usage (as done with mkv).