Add getter() function + Normalised Store example

[quandhz]

Branch: https://github.com/quandhz/resaga/tree/subscribe_value_getter
Resolve: #57

Run example npm run examples:normalise

getter(value, initialProps) => filteredValue;

Now we can subscribe to a value with your own getter function to filter out what we don't need.

Pseudo-code

// BookItem component
const CONFIG = {
  value: {
    title: {
      keyPath: ['BookPage', 'books'],
      getter: (books, initialProps) => books[initialProps.id].title,
    },
  },
};

Usage

<BookItem id={1} />
<BookItem id={2} />
<BookItem id={3} />

Each BookItem will subscribe to its book title based on the id passed in.

[codecov]

Codecov Report

Merging #58 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #58   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          27     30    +3     
  Lines         329    370   +41     
  Branches       68     85   +17     
=====================================
+ Hits          329    370   +41

Impacted Files	Coverage Δ
lib/internal/helpers/selectors.js	100% <100%> (ø)
...ib/internal/hoc/components/helpers/exportValues.js	100% <100%> (ø)
lib/internal/helpers/analyseConfig.js	100% <100%> (ø)	⬆️
lib/internal/hoc/components/SimpleResaga.js	100% <100%> (ø)	⬆️
lib/internal/hoc/withResaga.js	100% <100%> (ø)	⬆️
lib/internal/helpers/subscribeValue.js	100% <100%> (ø)
lib/internal/selectors.js	100% <100%> (ø)	⬆️
lib/internal/hoc/withSimplifyResaga.js	100% <100%> (ø)	⬆️
lib/internal/hoc/components/Resaga.js	100% <100%> (ø)	⬆️
... and 2 more

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