Extension selection doesn't seem to work properly
Consider:

`python main.py --profile=core --out-path=testloader --api="gl=" --generator=c --spec="gl" --extensions="GL_AMD_gpu_shader_int64"`

Works fine.

`python main.py --profile=core --out-path=testloader --api="gl=" --generator=c --spec="gl" --extensions="GL_AMD_gcn_shader,GL_AMD_gpu_shader_int64"`

Now I get an error saying GL_AMD_gpu_shader_int64 is invalid.

I think the problem is in glad/generator/generator.py. The generate method does:

``` python
        e = chain.from_iterable(self.spec.extensions[a] for a in self.api)
        for ext in extension_names:
            enforce(ext in e, 'Invalid extension "{}"'.format(ext), ValueError)
```

But I think it should be `e = list(chain.from_iterable(self.spec.extensions[a] for a in self.api))`. Changing it to this works properly.
