support for extra custom info to messages

I worked last few days on centralized translation system for web and mobile clients (android/ios). I have added one feature to this bundle that allowed me to make this task easier. I made support for custom data associated with Message object. There can be various use of this feature, in my case I used this extra data to store information about android translation id, ios translation id and ios translation domain. There are new methods in Message class: `getExtra(name)`, `getExtras()`, `addExtra(name, value)`

You can add extra info to Message in this way:

```php

/** @Extra(name="name",value="value")  */
$translator->trans(...);
```

In twig template:

```
{{ "some text"|trans|extra("name", "value") }}
```