Support --diff mode

[gavv]

When several inputs are provided, and --diff mode is enabled, run estimator for first input + Nth input instead of output + Nth input. This will measure latency between inputs instead of latency between output and each input.

TODO:

• [x] add support in Runner
• [x] add CLI option
• [x] update reporters: show sign
• [x] fix StepsLatencyEstimator
• [ ] fix CorrelationLatencyEstimator