Makes reference to another variable in the same translation file

Example:

**File :** _lang.en.yml_
- en:
- hi: "Hello world!!!"
- common:
- password: "Password"
- username: "Username"
- create: "Create"
- add: "Add"
- remove: "Remove"
- delete: "Clear"
- cancel: "Cancel"
- save: "Save"
- email: "E-mail"
- search: "Search"
- login: "Login"
- register: "Register"
- version: "Version"
- logged: "Logged in as **reference to common.username** "

python-i18n do this? referencing **logged** to **common.username**.

Start by reviewing the lang.en.yml example and the python-i18n translation-loading path. Determine how a value such as logged can reference common.username, and consider the existing six-comment discussion before defining completion as resolving that reference in the translated output.

Scritto dal modello di indicizzazione a partire dal testo della issue.