Support for cacheContext override

[mistic]

This PR give the cache-loader users the ability to override the cacheContext (something close to what is being done into the dll plugin) in order to generate the cache asset paths relatively to another path. This will make it possible to save the cache and re-use it in the CI for example or even ship it for other usages.

This also close/replace #37 and close #36

\cc @evilebottnawi

[codecov]

Codecov Report

Merging #49 into master will decrease coverage by 1.21%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   17.14%   15.92%   -1.22%     
==========================================
  Files           2        2              
  Lines         105      113       +8     
  Branches       15       17       +2     
==========================================
  Hits           18       18              
- Misses         73       79       +6     
- Partials       14       16       +2

Impacted Files	Coverage Δ
src/index.js	16.07% <0%> (-1.24%)	⬇️

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[codecov]

Codecov Report

Merging #49 into master will increase coverage by 40.97%.
The diff coverage is 45.83%.

@@             Coverage Diff             @@
##           master      #49       +/-   ##
===========================================
+ Coverage   17.14%   58.11%   +40.97%     
===========================================
  Files           2        2               
  Lines         105      117       +12     
  Branches       15       17        +2     
===========================================
+ Hits           18       68       +50     
+ Misses         73       42       -31     
+ Partials       14        7        -7

Impacted Files	Coverage Δ
src/index.js	58.62% <45.83%> (+41.31%)	⬆️

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[alexander-akait]

Looks good! Can we add couple simple tests? Because we can't have here tests and it is very bad

[mistic]

@evilebottnawi some testes were for that specific option!

[mistic]

@evilebottnawi this PR here is dependent on #50 has I've used webpack 4 to run the tests

[alexander-akait]

Good work

[alexander-akait]

Need rebase and CI should be green

[mistic]

@evilebottnawi done

[mistic]

CI is green 🎉