Add new accessor function getByKeys()

[spencerwi]

Added the getByKeys() method described in #28, and exercised it in some tests (and updated the README). Y'all's changelog appears autogenerated, so I didn't touch that, as a heads-up.

Sorry for the multiple "referenced a commit" over on the issue -- to keep things clean and down to a single commit, I kept force-pushing to my own fork repo (normally bad form, I know, but for a quick PR branch I figured it seemed alright).

Fixes #28

[codecov-io]

Codecov Report

Merging #30 into master will increase coverage by 1.2%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #30     +/-   ##
=========================================
+ Coverage    69.1%   70.31%   +1.2%     
=========================================
  Files           5        5             
  Lines         123      128      +5     
  Branches       19       20      +1     
=========================================
+ Hits           85       90      +5     
  Misses         31       31             
  Partials        7        7

Impacted Files	Coverage Δ
src/impl/domain.ts	73.91% <100%> (+0.77%)	⬆️
src/index.ts	61.53% <100%> (+2.35%)	⬆️

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[kabirbaidhya]

@spencerwi Awesome. Thanks for contributing 🥇 👍 🙏.

There are few changes I'd request before signing off:

1. Tweak the implementation for getByKeys - to return an identical "array" of values instead of an object. Please check the example I created in the issue New accessor function to retrieve a list of values by keys #28.

   This is the desired behavior:

   - const results = store.getByKeys(['a', 'b', 'sum']); // {a: 1, b: 2, sum: 3}
   + const results = store.getByKeys(['a', 'b', 'sum']); // [1, 2, 3]

   This gives you an ability to do this more intuitively:

   const [a, b, sum] = store.getByKeys(['a', 'b', 'sum']);

2. Update tests, docblocks and README as per the above change.

[spencerwi]

Sure, I'll make those changes. I think I misunderstood the original issue.

In terms of destructuring, this is legal in Javascript:

// assuming the store has {a: 1, b: 2, sum: 3, empty: null}
let {a, b, sum, empty, notFoundThing} = store.getByKeys(['a', 'b', 'sum', 'empty', 'notFoundThing']);
console.log(a) // 1
console.log(b) // 2
console.log(sum) // 3
console.log(empty) // null
console.log(notFoundThing) // undefined

[spencerwi]

I've made the changes you requested -- thanks for the feedback! Hopefully the PR in its current state better fits the desired use-case. 😄

[kabirbaidhya]

Great. LGTM 🎉 💯