Allow adding listeners to execution

[Arvengeance]

I would like to add listeners on every execution of robot test cases.

My use case: Adding a listener that does real-time logging in to Elasticsearch.

[Noordsestern]

There are more things of robot cmd api that could be published as well. Maybe, it is possible to provide argumentfiles and pass it on to the robot execution inside the webservice.

I will check it out.