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
Afaik there is no good reason to manually maintain linebreaks for prose, it just makes it more difficult to make edits. I don't know of any style guide for this. Our [html styleguide](http://contribute.jquery.org/style-guide/html/) doesn't mention it.