addInterceptor(interceptorFn) {
			if (typeof(interceptorFn) == 'function')
				this.interceptors.push(interceptorFn);
			return interceptorFn;
		}
Register an HTTP response interceptor function. Interceptor
functions are useful to perform default actions on incoming HTTP
responses, such as checking for expired authentication or for
implementing request retries before returning a failure.

@instance
@memberof LuCI.request
@param {LuCI.request.interceptorFn} interceptorFn
The interceptor function to register.

@returns {LuCI.request.interceptorFn}
The registered function.