Read only option

[mistic]

In some environments it useful to not write anything the cache and just using it in read-only mode ( for example in a CI environment or even in a production environment running the webpack build for some reason ).

This PR along with other already merged (#49) will gave us the ability to run cache-loader, with pre-built caches, in almost every environment we wanna use it.

/CC @evilebottnawi

[codecov]

Codecov Report

Merging #61 into master will decrease coverage by 2.02%.
The diff coverage is 37.5%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   58.47%   56.45%   -2.03%     
==========================================
  Files           2        2              
  Lines         118      124       +6     
  Branches       17       19       +2     
==========================================
+ Hits           69       70       +1     
- Misses         42       46       +4     
- Partials        7        8       +1

Impacted Files	Coverage Δ
src/index.js	56.91% <37.5%> (-2.07%)	⬇️

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[alexander-akait]

I think we need more test, but let's do this in other PR