add aggregated results to report aggregator and configurable thresholds to maven aggregator mojo

[Pfoerd]

Fixes #1068 .

Adds support for thresholds for aggregated mutation coverage, test strength and max. surviving mutants in multi module projects:

<plugin>
	<groupId>org.pitest</groupId>
	<artifactId>pitest-maven</artifactId>
	<version>${pit.version}</version>
	<configuration>
		//...
		<aggregatedTestStrengthThreshold>26</aggregatedTestStrengthThreshold>
		<aggregatedMutationThreshold>20</aggregatedMutationThreshold>
		<aggregatedMaxSurviving>6</aggregatedMaxSurviving>
	</configuration>
</plugin>

As this is added as a result of ReportAggregator#aggregateReport this can also be used in the pitest gradle plugins to configure thresholds for the aggregated metrics in the next step.

[hcoles]

Thanks