add cacheAddedFiles option

[mochiya98]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

Add option to allow cache the added files.
This fixes #99.
It also includes small improvements.

Breaking Changes

None.

Additional Info

Old cache files will be unavailable.

[codecov]

Codecov Report

Merging #109 into master will increase coverage by 2.33%.
The diff coverage is 95.23%.

@@            Coverage Diff             @@
##           master     #109      +/-   ##
==========================================
+ Coverage   83.09%   85.43%   +2.33%     
==========================================
  Files           2        2              
  Lines         142      151       +9     
  Branches       21       21              
==========================================
+ Hits          118      129      +11     
+ Misses         24       22       -2     

Impacted Files	Coverage Δ
src/index.js	85.33% <95.23%> (+2.35%)	⬆️

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[bravomartin]

This is great. It solves my use case, which is to combine it with responsive-loader. It takes a very long time since it generates multiple versions of each image. I was bummed when I realized it wouldn't emit the files on the second run.

[mochiya98]

Commitlint has been updated. and I've fixed npm audit issue.
Now ready to merge!
@evilebottnawi Could you review the PR please?

[bravomartin]

I'm eagerly awaiting for this PR to be merged.

It will make this plugin so much more useful since it will allow using it for caching assets when responsive-loader and other slow loaders.

[runemadsen]

We are waiting for this PR too! It is a bit odd that cache-loader does not cache images, and it took us a while to debug.