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
I've added inline examples. But, it's still confusing. Because, it lacks message context. Inlining the message as well would duplicate the below. What do you think?

I'm considering to revert this and mention: see example below instead.