v0.5.1

• Change encoded size prediction logic to estimate video stream size (or image size) only.
  This should be much more consistent than the previous method.
  Change crf-search, sample-encode result text to clarify this.
• Improve video size prediction logic to account for samples that do not turn out as 20s.
• Fix full-pass sample encode progress bar.
• Use label "Full pass" instead of "Sample 1/1" when doing a full pass sample-encode.
• Add VMAF auto model, n_threads & scaling documentation.