Fix 220: Create plural module

Borrow santhoshtr/CLDRPluralRuleParser.

Ref #220

- [x] Implementation (basically a wrapper);
- [x] Unit tests;
- [x] Build;
- [x] Functional tests;
- [x] Docs;

Let's cherry-pick a few of these and remove the rest, since the underlying method is already covered by the unit test for `pluralForm`.