feat(server): config.capturePort

Had a problem tunnelling karma server through ngrok (ngrok http port is 80).
I think we can keep default value to equal `DEFAULT_PORT` then we can remove check [here](https://github.com/karma-runner/karma/pull/1168/files#diff-ff5050d3c7f51fca90c4691a4599e852R36) and remove unused deps `port` [here](https://github.com/karma-runner/karma/pull/1168/files#diff-ff5050d3c7f51fca90c4691a4599e852R35).

Also it will be clear in documentation that `caputrePort` by default will be equal `port` than `null`.

Thanks!