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
Just noticed the linebreaks here for the first time, probably applies to existing docs. In other places we don't add line breaks to prose in markdown or html (unlike commit messages and code). See for example this file: https://raw.githubusercontent.com/jquery/jqueryui.com/v1.11.11/page/upgrade-guide/1.11.md