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;

Talking with Jörn, we decided to leave it as is. Both approaches (generating this piece or leaving the header untouched) have advantage and disadvantages. Generating it has the advantage of de-duplicating it. Although, it will make the grunt task more complex.