Add feature to skip unchanged images during refresh

[nerikosans]

Hello, thank you for publishing this great plugin.

This pull request fixes #14 , adding skipUnchanged option.

Implementation Summary

When skipUnchanged is true, it stores the SHA-256 hash value of each images to be converted. Like below:

// ImageminWebpWebpackPlugin.sourceVersions
{
  'assets/bicycle_2.webp': 'e3ab38b7bea9e13f7ec748019a92e45b8c13dbafcb612cb07ef59f6dc346a866',
  'assets/cookie-monster.webp': 'ef8a7672e385b4ab734129baf16db2597b19547580c9a88f9a1c81b5faa54953',
  'assets/images/folder.with.dots/inner-folder/bicycle_1.webp': '93cc700fbf54c93c9503166343c9f3d2bfcbc98d2049bc38972e8501d8c26c20'
}

Then during refreshes, conversions are skipped if the target image's hash is already stored.

Note: This implementation is inspired by an official example from webpack for monitoring changed chunks.
https://webpack.js.org/contribute/plugin-patterns/#changed-chunks

Experimental results

Experiments on test projects showed this change saves a considerable ratio of runtime. 🎉

Webpack 5: ~800ms → ~150ms

Original	With skipUnchanged

It detects and converts newly added/modified images:

Webpack 4: ~1300ms → ~350ms

Original	With skipUnchanged

[nerikosans]

Hi @iampava, would you kindly take a look at this? I would appreciate it so much!

[georgi-vdimitrov]

+1

[XenBG]

+1

[vdzhorov]

+1

[lmarkov]

+1

[gwustick]

+1

[deaniliev]

+1

[mladen-venkov]

+1

[Radnevec]

+1

[PlamBG]

+1

[smisra3]

@iampava Please review this. Seems like a really amazing feature to have.