Add a formatHsl formatter

Firstly, thanks for the great library!

With our usecase we want to be able to output color strings for hex, rgb, and hsl, though culori doesn’t currently support hsl. For now I’ve just written my own stringifier but it feels like this might make sense in the library itself.

Proposed solution

Following the existing formatter API, it would accept a color or string and then return hsl/hsla based on whether there’s an alpha less than 1.

culori.formatHsl('tomato')

I’m not super familiar with culori internals, but I see that hex/rgb handle the fixups from within the rgb color mode. Is there a way we can do the same thing with hsl? Is this something you would accept a PR for?

[danburzo]

Added a first pass at the implementation in #130. Currently h, s and l are rounded to the nearest integer. Both rgb() and hsl() color notations explicitly accept float values in CSS Color 4 but I’m not sure what the support story is for floats in the hsl() notation. If it’s as patchy as for rgb(), then I think it would be best to round the values in hsl as well. Otherwise, 2-digit precision floats sound better to me. Maybe there are some Web Platform Tests about this specifically…

[johno]

Thank you very much @danburzo. Your work and library are greatly appreciated!