WiP: Adding callbacks to StoreConnection

[Will-W]

flutter_redux has 3 optional callbacks to StoreConnection (onWillChange, onDidChange, and onInitialBuild). I have implemented one because I needed it, Issue #34 implies a need for another.

I'm happy to go ahead and implement all 3, but before I did I wanted to check:

1. Are you happy with the de# principle?
2. What would need to be done to the implementation of onDidChange in this PR to make it suitable for merging? The main thing I could see would be a test.

[codecov-io]

Codecov Report

Merging #37 into master will not change coverage.
The diff coverage is 87.5%.

@@           Coverage Diff           @@
##           master      #37   +/-   ##
=======================================
  Coverage   85.71%   85.71%           
=======================================
  Files           1        1           
  Lines          35       42    +7     
=======================================
+ Hits           30       36    +6     
- Misses          5        6    +1

Impacted Files	Coverage Δ
lib/flutter_built_redux.dart	85.71% <87.5%> (ø)	⬆️

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[kdela]

@davidmarne @Will-W Any estimations when It would be merged?