Support for message key prefix

This feature simplify writing of translate keys in templates. Example of Nette template:

```
{block #content}
{translatePrefix 'messages.example.'}
    <h1>{_title}</h1>
    <h2>{_subtitle}</h2>
    <p>{_some_text}</p>
{/translatePrefix}
{/block}
```

messages.en_EN.neon file:
```
example:
    title: "Title of the page"
    subtitle: "Subtitle of the page"
    some_text: "Lorem ipsum sil dolor amet .."
```

You appreciate this, when you'r preparing large Nette templates for translation. Writing/copying same prefix of keys is really annoying.