feat(directive): add possibility to mix translation interpolation with other text in element body

This adds possiblity to use like this:
```html
<span translate>Hello {{translation_id}}!</span>
```
Given that $scope.translation_id = "CHARACTER_NAME", which translates into "Muppet", this would become
```html
<span translate>Hello Muppet!</span>
```

Closes #461