Provide contrast ratio function

[Hadley Wickham]

like colorspace::contrast_ratio("#dc3545", "#f8f9fa")

Not exactly related to the core purpose of the package, but when generating colours it is very useful to be able to check their accessibility.

[Emil Hvitfeldt]

Are you thinking contrast_ratio()? 😁
https://emilhvitfeldt.github.io/prismatic/reference/contrast_ratio.html

Also have best_contrast() helper which is vectorized by first argument
https://emilhvitfeldt.github.io/prismatic/reference/best_contrast.html

[Emil Hvitfeldt]

Closed as duplicate of #18

[Hadley Wickham]

I can't believe I already filed this issue and you fixed it 😆