Add support for structural similarity index measure (SSIM)

[jihchi]

References

• Wang, Z.; Simoncelli, E. P. (September 2008). "Maximum differentiation (MAD) competition: a methodology for comparing computational models of perceptual quantities" (pdf)
• Recommendations when using SSIM comparison from jest-image-snapshot
• https://en.wikipedia.org/wiki/Structural_similarity
• SSIM.js

[jihchi]

There is already a crate/CLI called dssim where implemented the SSIM.