Add the ability to read stubs

[gianiaz]

If you install the phpstan/phpstan-doctrine plugin it adds stubs for making Doctrine EntityRepository generic, like this one:

https://github.com/phpstan/phpstan-doctrine/blob/master/stubs/EntityRepository.stub

It would be nice if the plugin could read these files for making phpstorm aware that a call to a find on the repo could return only null|myEntity.