Refactor enhancer store subscriber and history listener

[aweary]

I had some trouble testing changes made to one of these callbacks in https://github.com/FormidableLabs/redux-little-router/pull/240, so here's a small refactor that makes them a bit more testable. This PR:

• Stops defining the callbacks as inline arrow functions and implements createStoreSubscriber and createHistoryListener instead. These functions accept some constant dependencies (like store) and return functions that can be called with the dynamic values like currentMatcher

• Adds tests for both of those new functions

I'm not sure it's the best solution but it seems sufficient to me 🤷‍♂️

[codecov[bot]]

Codecov Report

Merging #242 into master will increase coverage by 0.42%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #242      +/-   ##
==========================================
+ Coverage   98.44%   98.87%   +0.42%     
==========================================
  Files          19       19              
  Lines         258      266       +8     
==========================================
+ Hits          254      263       +9     
+ Misses          4        3       -1

Impacted Files	Coverage Δ
src/enhancer.js	100% <100%> (+5.55%)	:arrow_up:

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