Add support for requestInterceptor / responseInterceptor.

- Display mutated requests from request interceptor in curl output in UI.
  Put this behind showMutatedRequest flag so that the mutation can be
  silent.
- Document requestInterceptor, responseInterceptor and showMutatedRequest
  in README.md
- Add tests

src/core/components/operation.jsx

@@ -263,6 +263,7 @@ export default class Operation extends PureComponent {
                     request={ request }
                     tryItOutResponse={ response }
                     getComponent={ getComponent }
+                    getConfigs={ getConfigs }
                     specSelectors={ specSelectors }
                     specActions={ specActions }
                     produces={ produces }