Multiple config files

In case I have a big project with a lot of definitions in different folders I'd like to have a config file (codegen.yaml) for each of these folders.

Now we can do:
```
graphql-codegen --config codegen.yaml
```
It would be great to have:
```
graphql-codegen --config src/modules/module1/codegen.yaml,src/modules/module2/codegen.yaml
```
or:
```
graphql-codegen --config src/modules/**/codegen.yaml
```

What do you think?