[META] Introduce new formats

We would like to support more export formats. So far, we have discussed supporting WordDoc with the PHPWord proejct (https://github.com/PHPOffice/PHPWord) and Epub with the PHPePub project (https://github.com/Grandt/PHPePub).

Hopefully, all of these formats will be provided by libraries that support a conversion from HTML to destination format. That will allow us to create an engine per export format e.g. WordDocEngine, EpubEngine, etc.

We will reference tasks for specific formats with this meta issue.