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
True, `/` cannot be used. We should have that documented.

> why slash instead of a dot?

I'm simply being consistent with cldr paths, which is using slashes `/` for consistency with these [ldml paths](http://www.unicode.org/reports/tr35/#Count_Fallback_normal).