Create merged report for multi-project scenarios

Hi,

My name is Chanh and I'm an engineer at LinkedIn. We use play-scala at LinkedIn and have been trying to leverage Jacoco so that we can get a nice report for each play app. We found that Jacoco doesn't currently generate a merged report for multi-project apps so we added some work internally to support this. 

This pull request takes a stab at contributing the work back into jacoco4sbt. I would really appreciate your feedback on this. I am quite new to SBT so please be gentle :) 

Thanks,
Chanh