Adding linked translations

[mochetts]

This adds the enhancement requested on issue #26.

The idea is the same as Angular translate.
After the string has been translated with the current mechanism, it will replace each occurrence of a link translation (linked translations are preceded by "@:") and replace them by the corresponding translation.

So if you have the following locales:

en: {
    message: {
      the_world: 'the world',
      hoge: 'hoge',
      linked: 'Hello @:message.hoge!, isn\'t @:message.the_world great?'
    }
}

The result of executing $t('message.linked') will be Hello hoge!, isn't the world great?

[coveralls]

Coverage increased (+0.4%) to 91.355% when pulling 21e5957 on mmochetti:linked-translations into e7c3d23 on kazupon:dev.

[kazupon]

awesome!! 💯
Thanks !!