Plural: Create generator method

Fixes #335 

- [x] Implement
  - [x] Drop formatPlural (use messageFormat instead) - it doesn't impact other modules (e.g, unit).
- [x] Plural generator
- [x] Functional tests
- [x] Docs
- [x] eemeli/make-plural.js license. [ISC is MIT equivalent](http://en.wikipedia.org/wiki/ISC_license).
- [x] Browser support