Added possibility to stop the forked runner process

Hi Guys,

To keet the story short :) :
We are using sonar runner embedded in our IntelliJ sonar plugin. We almost finished the work for that, but we found out that there is a new API build during that time. 
I  did an evaluation for the new API for the sonar runner and we would like to use it. (https://github.com/gshakhn/sonar-intellij-plugin/pull/38).

I noticed that we ForketSonarRunner cannot be stopped once is started. In our IntelliJ--run-local-analysis-sonar integration we offer the possibility to stop the runner once it was started. 

Please my contribution which adds the possibility to stop the sonar runner. I tried to not bring to much impact on the "current interface". Woud be really great if we would have this feature since then we could finalize the run-local-analysis with the newest API. 

Please review/feedback if this fix can be pushed to sonar runner. I would be glad to tweak if necessary.

Kind regards,
Alpar