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

test/core/plugins/spec/actions.js

@@ -93,6 +93,54 @@ describe("spec plugin - actions", function(){
       })
     })
 
+    it("should pass requestInterceptor/responseInterceptor to fn.execute", function(){
+      // Given
+      let configs = {
+        requestInterceptor: createSpy(),
+        responseInterceptor: createSpy()
+      }
+      const system = {
+        fn: {
+          buildRequest: createSpy(),
+          execute: createSpy().andReturn(Promise.resolve())
+        },
+        specActions: {
+          executeRequest: createSpy(),
+          setMutatedRequest: createSpy(),
+          setRequest: createSpy()
+        },
+        specSelectors: {
+          spec: () => fromJS({}),
+          parameterValues: () => fromJS({}),
+          contentTypeValues: () => fromJS({}),
+          url: () => fromJS({})
+        },
+        getConfigs: () => configs
+      }
+      // When
+      let executeFn = executeRequest({
+        pathName: "/one",
+        method: "GET",
+        operation: fromJS({operationId: "getOne"})
+      })
+      let res = executeFn(system)
+
+      // Then
+      expect(system.fn.execute.calls.length).toEqual(1)
+      expect(system.fn.execute.calls[0].arguments[0]).toIncludeKey("requestInterceptor")
+      expect(system.fn.execute.calls[0].arguments[0]).toInclude({
+        responseInterceptor: configs.responseInterceptor
+      })
+      expect(system.specActions.setMutatedRequest.calls.length).toEqual(0)
+      expect(system.specActions.setRequest.calls.length).toEqual(1)
+
+
+      let wrappedRequestInterceptor = system.fn.execute.calls[0].arguments[0].requestInterceptor
+      wrappedRequestInterceptor(system.fn.execute.calls[0].arguments[0])
+      expect(configs.requestInterceptor.calls.length).toEqual(1)
+      expect(system.specActions.setMutatedRequest.calls.length).toEqual(1)
+      expect(system.specActions.setRequest.calls.length).toEqual(1)
+    })
   })
 
   xit("should call specActions.setResponse, when fn.execute resolves", function(){