extended instrumentation goal to allow for additional user defined instrumentation arguments

I was a little unsure on how to test my extension of the ```AbstractInstrumentationMojo```, so I kept the changes to a minimum there and extracted the actual "parsing" to a separate ```InstrumentationArgumentParser``` class, which is unit tested completely.

I would be happy to also throw in some coverage for the ```AbstractInstrumentationMojo```, but it seems that there is no real unit test setup yet. I guess some refactoring might be involved to make this component testable.

Any ideas/suggestions?

Also I would like to have some feedback about the way I merge in the customer defined arguments, because the user is now able to override plugin defined configurations manually.