Introduce string translation

[jessebeach]

This is not a small undertaking.

We need:

1. A facility to translate strings defined in tests
2. .po files for languages
3. An initiative to encourage contributors to translate strings.

[nschonni]

Might also want to take a look at https://github.com/SlexAxton/Jed or other client side options to be CLI friendly

[kevee]

I like the idea of using PO files, since there's a lot of great tools out there for managing them already. We could also use grunt-po2json to create JSON files for individual languages, or all languages.

[hannolans]

For the moment we can add the translations in the file https://github.com/quailjs/quail/blob/master/src/resources/tests.yml
Am I right?