Permit automatic exclusions aggregation strategy

[dlh3]

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):

<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).

[dlh3]

I will try to find the time to contribute with a pull request.

[dlh3]

Added commit to implement requested feature. Includes an additional test case covering the change.