Make processing unit test results optional

[kragniz]

In our deployment of jenkins + phabricator we'd like to let jenkins handle unit test results, but to not get phabricator involved (to avoid user workflows becoming dependent on it, and to reduce load on phabricator).

Basically execution of this line should be controlled by a new option:
https://github.com/uber/phabricator-jenkins-plugin/blob/8b42c188c4112972ac0cb24f5f142573412c8f7c/src/main/java/com/uber/jenkins/phabricator/PhabricatorNotifier.java#L278

Is this something which would be accepted upstream?