Add mixin-like approach of connecting custom middleware to Store

[soxjke]

This is intended to close issue #3

TODO:

• Add unit tests coverage
• Add docs

[codecov]

Codecov Report

Merging #8 into master will increase coverage by 0.39%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master      #8      +/-   ##
=========================================
+ Coverage   95.91%   96.3%   +0.39%     
=========================================
  Files           3       2       -1     
  Lines         392     298      -94     
  Branches       32       0      -32     
=========================================
- Hits          376     287      -89     
+ Misses         16      11       -5

Impacted Files	Coverage Δ
Redux-ReactiveSwift/Tests/StoreSpec.swift	100% <0%> (ø)	⬆️
Redux-ReactiveSwift/Tests/CallSpy.swift	80.7% <0%> (+2.32%)	⬆️

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