Possibility to setup multiple locales look up

[AlexMost]

Addressing this PR - #70

So, to be able to provide multiple places for the translations lookup, I decided to add simple useLocales method to the existing API. This one is mostly implemented here - #70. Another part of the PR includes some code refactoring.

Example:

import { useLocales, addLocale } from 'c-3po';

addLocale('en', obj1);
addLocale('en_US', obj2);

useLocales(['en_US', 'en']);

@Alxpy @opyh

[codecov]

Codecov Report

Merging #71 into master will decrease coverage by 0.67%.
The diff coverage is 96.51%.

@@            Coverage Diff            @@
##           master     #71      +/-   ##
=========================================
- Coverage   75.97%   75.3%   -0.68%     
=========================================
  Files           3       4       +1     
  Lines         587     741     +154     
  Branches      101     121      +20     
=========================================
+ Hits          446     558     +112     
- Misses        141     183      +42

Impacted Files	Coverage Δ
src/utils.js	77.82% <100%> (-0.27%)	⬇️
src/index.js	80.25% <100%> (+2.88%)	⬆️
src/config.js	68.27% <94.44%> (ø)

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[oleksandr-kuzmenko]

nice 👍