Support class component (HOC)

[halitogunc]

This PR enables react-wait to use in a class component via injected prop #8 and also contains some refactoring stuffs that I wanted to request your comments. @f

~~I tested all functionality on my machine and it worked, but I didn't prepare test cases yet. This is why it is work in progress.~~ Since tests were added, it is not the case anymore.

Waiting for the review :shipit:

[codecov[bot]]

Codecov Report

Merging #11 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #11   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      5    +3     
  Lines          23     35   +12     
  Branches        2      4    +2     
=====================================
+ Hits           23     35   +12

Impacted Files	Coverage Δ
src/context.js	100% <100%> (ø)
src/provider.js	100% <100%> (ø)
src/injectWaiting.js	100% <100%> (ø)
src/hooks.js	100% <100%> (ø)

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[halitogunc]

just sending you a kindly reminder @f