X-Forwarded-For header stripping option for nghttpx

Introduces a new option `--strip-incoming-x-forwarded-for` that will remove any X-Forwarded-For header contained in an incoming request. This option can be used in combination with `--add-x-forwarded-for`. The following image captures the combinations possible and the expected downstream request header:

![stip-x-forwarded-test-results_trimmed](https://cloud.githubusercontent.com/assets/6571445/4771266/4e221146-5b8d-11e4-9986-32d4a6d219b3.PNG)