Permit automatic exclusions aggregation strategy

It would be great to have an aggregation strategy that would omit files which were included in other aggregations.

Example (assume files A.css through H.css):
```xml
<aggregations>
	<aggregation>
		<includes>
			<include>**/A.css</include>
			<include>**/B.css</include>
			<include>**/C.css</include>
		</includes>
		<output>${project.build.outputDirectory}/A-C.css</output>
	</aggregation>
	<aggregation>
		<includes>
			<include>**/F.css</include>
			<include>**/G.css</include>
		</includes>
		<output>${project.build.outputDirectory}/F-G.css</output>
	</aggregation>
	<aggregation>
		<includes>
			<include>**/*.css</include>
		</includes>
		<excludes>
			<exclude>**/H.css</exclude>
		</excludes>
		<autoExclude>true</autoExclude>
		<output>${project.build.outputDirectory}/D-E.css</output>
	</aggregation>
	<aggregation>
		<includes>
			<include>**/*.css</include>
		</includes>
		<autoExclude>true</autoExclude>
		<output>${project.build.outputDirectory}/H.css</output>
	</aggregation>
</aggregations>
```

This would be a great convenience for the case where multiple special aggregations are needed (like A-C and F-G).