Allow to define a delay for intercepted requests only

[roobre]

The current API allows to inject both error codes, and additional latency to requests. The former is done by means of errorBody and errorCode, and the latter by those of averageDelay and delayVariation. For injecting errors, it is also possible to specify a probablity of how many requests will error using errorRate.

Currently, it is however not possible to cause only requests that are intercepted to return an error to also have an increased latency.

This is not the same as causing only a fraction of requests to be delayed, which is covered in #196.