Create a state stream as the action property being accessed

[creasty]

Why

If the state weren't subscribed from anywhere, actions sent to Reactor would be simply ignored.

func bind(reactor: Reactor) {
        reactor.action.onNext(.load)
}

What

Touch state property in action property to ensure the initialization of the state stream

[creasty]

cc @hedjirog

[codecov-io]

Codecov Report

Merging #49 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
+ Coverage   91.91%   91.95%   +0.04%     
==========================================
  Files           7        7              
  Lines         198      199       +1     
  Branches       26       26              
==========================================
+ Hits          182      183       +1     
  Misses         14       14              
  Partials        2        2

Impacted Files	Coverage Δ
Sources/ReactorKit/Reactor.swift	79.16% <100%> (+0.29%)	⬆️

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[devxoul]

Oh my god. I have no idea why I haven't try this approach. I really love this. Thanks! (cc. @innocarpe)

By the way, there is a generic subclassing bug(#30). I'll fix that after merging this. Thanks again!