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
I've updated origin/master. But, the it still show up to me also. Although:

It doesn't show up to me running `git diff origin/master..origin/fix-265-message-format`. So, a github delay/issue?