Message: Support ICU MessageFormat using slexaxton/messageformat.js

:boom: WIP...

- [x] Implement #265
- [x] Plural generator
- [x] Functional tests
- [x] Docs
- [x] slexaxton/messageformat.js license
- [ ] use slexaxton/messageformat.js w/ new license once tagged
- [x] example

This will be replaced with a tag once the liencese change is tagged, right?