[FEATURE] Support for defining indexes in templates

Enables the user to add indexes to the structure documents.

See [here](https://github.com/sulu-cmf/SuluSearchBundle/tree/develop#mapping-structure-documents) for more information.

Releated to the development of the [SuluSearchBundle](https://github.com/sulu-cmf/SuluSearchBundle)

__Tasks:__

- [ ] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | 
| BC Breaks     | no
| Doc           | 
@dantleech Since this is only in the cache, wouldn't you add this to the constructor? Probably a little bit faster, and would be more consistent. /cc @wachterjohannes