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 ([debeaf4][])
- [ ] use slexaxton/messageformat.js w/ new license once tagged
- [x] example

[debeaf4]: https://github.com/SlexAxton/messageformat.js/blob/debeaf4/LICENSE
The doc at this point works more as a reference than a tutorial, true. I'm referencing https://github.com/SlexAxton/messageformat.js/ at the end then. Please, just let me know if you want me to change this last sentence.