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][])
- [x] example

[debeaf4]: https://github.com/SlexAxton/messageformat.js/blob/debeaf4/LICENSE
Regarding explanation of how messages are stored, I'd add a paragraph below this one, something like:

> The provided messages are stored along side other cldr data, under the "globalize-messages" key. This allows Globalize to reuse the traversal methods provided by cldrjs [aka cldr-traverse, maybe]. You can inspect this data using `cldrjs.get("globalize-messages")`.