Add support for threaded/parallel generation of webp

[aaronmurniadi]

Hi, this PR introduces parallel processing for the generation of webp images.

Below is my local test results building my site in Macbook Pro M2 (2022) using regenerate: true

Run command:

$ bundle exec jekyll serve --profile

Build time comparison:

v1.0.0	v1.0.1 (threads: 0/1)	v1.0.1 (threads: 4)
done in 367.518 seconds.	done in 365.473 seconds.	done in 106.948 seconds.

That's~70% faster build time! 🚀

The tests was done by building my GitHub pages site. Most of the images there are quite large, hence the long build times 😁

• https://aaronmurniadi.github.io/photography

[aaronmurniadi]

Anyway, seems like this repository is abandoned?

For everyone who's interested in using my version of the jekyll-webp before this is merged, you can use it like this:

...
group :jekyll_plugins do
   ...
  gem "jekyll-webp", :git => "https://github.com/aaronmurniadi/jekyll-webp.git"
end
...

I'm able to cut my site build time in half:

Also, don't forget to add this in your _config.yml file:

webp:
  ...
  threads: 4
  ...

Cheers!