Add get_context() to prepare a context object

[cpd67]

Hello! This PR refs #104. It adds a new get_context() method on the GraphqlWsClient which can be overridden to inject values into the context. Usage would be similar to how you add things to a Django view's context:

def get_context(self):
    context = super().get_context()
    context.user = self.scope.get('user')
    context.other_attr = 'test'
    return context