Persist cache in Netlify

[zeroby0]

Just like #51, but in Netlify cache.

It seems you can manage Netlify's cache only via a Netlify plugin, but to build such a plugin, co-operation from this plugin is necessary. Specifically,

• A way to find out the output directory
• And more importantly, it would require this plugin to not process an image if the processed image file is already in the outputDir
• Include the options.sharpOptions in calculating the image hash

Then creating a Netlify plugin would be as simple as:

• Copy files in _site/img after build
• Restore files from cache to _site/img before build

Unless there is a reason to not re-use files in _site/img, can we please do this? :)

Edit: Wait, I think this would solve #51 too! Okay, I'm gonna start work on it.
Edit: It's done, I'll start PR in a few minutes if everything is okay.

[zeroby0]

It's done! 🎉

https://github.com/zeroby0/netlify-plugin-11ty
https://www.npmjs.com/package/netlify-plugin-11ty