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 design in 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?