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
Replying to @jzaefferer's comment on the commit 309ea0dd240560b6938114947651ce051ab0d71d in here, or it would get lost with a rebase.

> How do you test this demo? I checked our this branch, ran bower install inside the example directory, then get an error when running it on Globalize.loadMessages, since the Globalize version that bower installed doesn't have that method.

This demo is supposed to work when this change is landed and published. So, to test it now during development you'll have to update the bower installed globalize/dist files yourself (e.g., `rm -Rf bower_components/globalize/dist && cp -a ../../dist bower_components/globalize/`)