Specific `namingConvention` option for `typescript-react-apollo` generated documents

**Is your feature request related to a problem? Please describe.**
We would like to have a specific naming convention for documents generated by `typescript-react-apollo`.

Right now they fall under the category `typeNames` when it comes to the `namingConvention` config. So for example we cannot use SCREAMING_SNAKE_CASE for documents (constants), but keeping PascalCase for types.

**Describe the solution you'd like**
I would propose having a 3rd key available for `namingConvention`. So it would be `typeNames`, `enumValues` AND `documents` (for example)

**Describe alternatives you've considered**
Just keeping the same naming convention for documents and types. It breaks our style guideline, still it is not a showstopper.